OPTIMUM UTILIZATION OF GROUND WATER IN KOBO VALLEY, EASTERN AMHARA, ETHIOPIA A Thesis Presented to the Faculty of the Graduate School of Cornell University in the Partial Fulfillment of the Requirements for the Degree of Master of Professional Studies (MPS) By Abrham Melesse Endalamaw August 2009
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OPTIMUM UTILIZATION OF GROUND WATER IN KOBO VALLEY,
EASTERN AMHARA, ETHIOPIA
A Thesis
Presented to the Faculty of the Graduate School
of Cornell University
in the Partial Fulfillment of the Requirements for the Degree of
Master of Professional Studies (MPS)
By
Abrham Melesse Endalamaw
August 2009
ABSTRACT
Shortage of precipitation in Kobo valley limits the production of vegetables during dry
periods and the yield of cereals in the rainy periods. Irrigation from ground water
could enable farmers to cultivate more than once a year. Since pumping has an effect
on the ground water resources availability, effective management of water resources
using reliable calculation of historical groundwater balances at local and sub-
watershed scales is required (Kendy et al 2004). We used CropWat 4 Window to
determine PET of the area and the Crop Water Requirement (CWR) of onion, tomato
and pepper, which are cultivated using irrigation during dry months; T-M and simple
water balance equations were used to quantify annual recharge to the water table and
water table status under different irrigation scenarios. Although irrigation from the
groundwater could ensure the food security of the area, different water management
scenarios showed that the ground water table will be declining as a result. Recharge
and water table calculations show that irrigation increases the recharge to the water
table but at the same time reduces the overall water table depth due to pumping. Water
table depth will not be depleted if irrigation follows the CWR of vegetables.
Calculations for future water table levels indicate that, if the current irrigation rate is
extended across all of the irrigable land in the area, the water table level will fall by 2
m per year. To protect against further water table decline, flashfloods should be
captured and used to recharge to the ground water.
KEY Words: Recharge, water table, ground water balance, irrigation, crop water
requirement, Kobo Girana Valley Development Project, Kobo, Ethiopia
iii
BIOGRAPHICAL SKETCH
The author was born at Woldia town, North Wollo Zone of the Amhara Regional State
on February 23, 1983. He attended his elementary and junior secondary education at
Sanka elementary and junior secondary school. After completion of elementary and
junior secondary education, he attended high school at Woldia Senior Secondary.
The after a successful completion his high school study, he joined Arba Minch Water
Technology Institute, currently named as Arba Minch University, in 2000/2001. He
joined the department of Meteorology Science and graduated with a B.Sc degree in
Meteorology science in 2005.
Right after graduation in 2005, he began working at Arba Minch University as a
Graduate Assistant from 2005 to 2007 and as an Assistant Lecturer from 2007 to the
start of this study. The author was working in different management positions in the
department of Meteorology in addition to teaching.
He has research experience in the fields of meteorology, hydrology and agriculture in
his future career. The impact of climate change on water resource and agricultural
production is of the main interest of the author. He is interested to continue his PhD
study as fast as possible in water resource topics.
iv
“This work is dedicated to my family, friends and who loved me.
Special dedication goes to my mother Ertibam Alemu”
v
ACKNOWLEDGEMENTS
First I would like to thank Cornell University, Bahir Dar University and IWMI’s C19
for the financial support during this work.
I also thank Professor Tammo S. Steenhuis who provided me with invaluable ideas
and advice over the course of this research.
I thank Dr. Amy S. Collick, who was the one that made this work successful by
providing all she had to share with me.
I would also like to thank Ato Adinew Abate, Manager of KGVDP, who arranged for
my access to facilities and written documents in the KGVDP office, in addition to his
advice. Special thanks is also due to Girma Takele, Abera Getinet, Midgam Adinew,
(6) crop water requirements. Also, the model can be applied to estimate the irrigation
schedule for each crop with 5 different options: in different irrigation management
scenarios defined by irrigation manager, irrigation set at below or above critical soil
( ) ( )( )dsn eeVGRET −++Δ
+−+ΔΔ
= 20062.0136.15γ
γγ
23
depletion (% RAM), irrigation set at fixed intervals per crop growing stage, irrigation
set at deficit irrigation, and no irrigation. Afterwards, the CROPWAT model can
simulate the on-farm crop water balance, including: irrigation times, dates and depths.
Growing period and pattern
The growing period and pattern of the area is determined by using the ratio of the
average monthly rainfall to the average monthly potential evapo-transpiration. For the
determination of the crop growing period and growing pattern, critical ratio values
were used as recommended by the FAO. It states that, for rain-fed agriculture, the area
is double growing if the ratio has two peaks with a value above one in different
periods in the year, single growing if the ratio has one peak with a value above one in
a year or no growing period if the ratio has no peaks with a value above one in a year.
Ground water table computation
The ground water recharge and ground water table level are calculated using the
application of Thornthwaite Mather (T-M) procedure and a simple water balance
equation that balances recharge and pumping. The equation uses monthly /daily
potential evaporation and precipitation. The moisture status of the soil depends on the
previous day moisture content (AW), the difference between precipitation and
potential evapotranspiration and the available water capacity (AWC) of the soil. The
AW is calculated by two different methods depending on whether the potential
evaporation is greater than or less than the cumulative precipitation.
Case 1:
For the months that the potential evaporation is in excess of the precipitation (i.e., the
soil is drying out) the AW at a given time t is given by the formula (Steenhuis and Van
Der Molen. 1986), viz:
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………………………………………………… (5)
Where: AW t = the available water at time t (cm); AW t-Δt = the available water at time
t-Δt (i.e., previous month; cm); PET = cumulative evaporation over time period t (cm);
AWC = the available water capacity of the soil (cm) and P = precipitation over time
period t (cm).
But in the case of irrigation application, the moisture status of the soil depends on the
amount and the time of irrigation applied other than the PET and precipitation.
Therefore equation 6 will be modified to account irrigation factor in the soil moisture,
viz:
……………………….…………………… (6)
Case 2:
For months when precipitation is in excess of the potential evapotranspiration, (i.e.,
the soil is wet) the AW at a given time t is given by the formula:
…………………………………………………… (7)
And again in the case of irrigation application, the moisture status of the soil depends
on the amount and the time of irrigation applied other than the PET and precipitation.
Therefore equation 6 will be modified to account irrigation factor in the soil moisture,
viz
……………………………………………... (8)
Hence the final soil moisture at the root zone is
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……………………………………………. (9)
Finally recharge to the ground water table is estimated by the equation:
………………………………. (10)
Therefore the general ground water balance equation for an unconfined aquifer is used
to estimate the ground water table level when irrigation is applied. The ground water
balance equation is given as:
…………………………………………………………………… (11)
Where, I = Inflow (cm) during time Δt, O = Outflow (cm) during time Δt and Δw =
change in water level (cm).
Considering the various inflow and outflow components, the ground water balance
equation for a time period Δt is given as:
………………….………………………… (12)
Where; Ri = Recharge from Rainfall, Rr = recharge from field irrigation, Et=
Evapotranspiration, Tp = draft from ground water, Se =Influent recharge to rivers
(Base flow to the river), ΔS = change in ground water storage
Base flow to the river (Se) is estimated by Darcy’s law as:
……………………………………………………………………….. (13)
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Where Q is the discharge or flow rate (cm3/month), K is hydraulic conductivity
(cm/month); A is the cross sectional area (cm2); Δh is the head difference and Δ l is
the distance from the well to the river. Hence, the depth of base flow per time is
calculated by dividing equation 13 by the area. This gives:
………………………………………………………………. (14)
Where, Ht is the height after time t, HD is the height from the river to water table level
and l is the horizontal distance from the river.
Finally, the ground water table height below the ground can be estimated elevation
using simple water balance equation that balances the recharge, discharge and
pumping of ground water.
………………………… (15)
Where Ht and Ht-Δt are ground water height below the ground at times t and t-Δt
respectively. Ai and AT are irrigated and total irrigable areas respectively.
Therefore, equating 14 and 15 gives,
……………... (16)
Equation 16 is used to estimate ground water table level under different scenarios.
Hence the decline of the water table can be calculated by the formula
…………………………………………………………………… (17)
27
Where ………………………………………………………….. (18)
Assumptions
It is assumed that the ground water table level before irrigation was applied was at
equilibrium state, i.e. the recharge to the ground water and the base flows are equal.
We also take the average ground water table as 18m below the surface of irrigated
farm land.
For the development of future water table depth calculations, the daily rainfall from
1997 to 2007 is used for the period 2008 to 2018.
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CHAPTER FIVE 5. Result and Discussion Potential evapotranspiration
The potential evapo-transpiration (ETO) of the area is computed by CropWat 4
window software, which uses the Penman-Monteith formula calculating ETO from
temperature (minimum & maximum), wind speed at two meters above the surface,
solar radiation and relative humidity data. As can be seen from Table 1, the highest
PET occurs during May and is about 6 mm/day or 186 mm/month. The average PET
of the area is 5 mm/day or 147 mm/month. The average annual PET of the area is
1799 mm. Comparison of the mean monthly rainfall and PET reveals that, for the
maximum crop production in the area, irrigation is the most important parameter. As
seen from Table 1 and Figure 1, except for the months from mid June to August, a
substantial amount of water is needed to fill the evapo-transpiration needs of different
crops.
Table 1 : Daily and mean monthly PET of Kobo computed by CROPWAT 4 Window
Month PET (mm/day) PET (mm)Jan 3.91 121 Feb 4.44 124 Mar 5.1 158 Apr 5.53 166 May 5.99 186 Jun 5.86 176 July 5.34 166 Aug 4.69 145 Sept 4.48 134 Oct 4.4 136 Nov 4.27 128 Dec 3.82 118 Avg 4.9 147 Total 1,759
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Figure 1: Mean monthly precipitation and potential evapo-transpiration of Kobo, where PET is computed by the Penmann-Montheth equation
Growing pattern of the area
The assessment of the growing pattern of the area using the ratio of rain fall to PET
reveals that, unless supplementary irrigation is supplied, there is only one cropping
season. As seen from Table 2 and Figure 2 there is only one area with ratio greater
than 0.5 which is from mid June through August. Although there is another peak from
March through June, the value is less than 0.5. This shows that unless irrigation is
supplied to the area, crop production will remain limited to one season. According to
the assessment of the yield reduction during these months using the CropWat soft
ware, the yield would be reduced by more than 50% if irrigation was not added for the
currently cultivated vegetables using ground water irrigation, Table 4. These crops
include onion, tomato and pepper. Hence, if rain-fed agriculture is concerned, the area
is characterized by single growing area. But, if the potential ground water resource is
used, the area can produce more than two times a year. Since 2005 the area has been
producing twice a year with the aid of ground water irrigation during the moisture
stressed periods. Comparisons of the crop water requirement for the three commercial
30
vegetables and the actual ground water delivered to the crops show that the amount of
water extracted is more than the crop water requirement.
Table 2: Ratio of mean monthly Rain Fall and Potential Evapo-Transpiration
Figure 2: Crop growing Pattern of Kobo estimated from the ratio of the areal rainfall to PET.
Irrigation and Field water balance under different management scenarios
Irrigation is one of the key factors affecting whether actual ET is close to the potential
rate. Components of a soil water balance as influenced by different irrigation
Month RF (mm/month) PET (mm/month) RF/PET Jan 19 121 0.16 Feb 7.26 124 0.06 Mar 30 158 0.2 Apr 64 166 0.42 May 36 186 0.22 Jun 13 176 0.09 Jul 172 166 1.22
Aug 231 145 1.65 Sep 48 134 0.36 Oct 49 136 0.35 Nov 25 128 0.2 Dec 29 118 0.25
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schedules in the three common vegetables (onion, tomato and pepper) grown during
the dry periods is shown in Table 4. Crop evapotranspiration (Etc) under different
treatments (i.e. without irrigation, irrigation with the current pumping rate and
irrigation recommended by CropWat 4 Window) for each vegetable varies among the
stages of development. Table 3 explains the effect of irrigation management scenarios
on the field water balance and the crop evapotranspiration. The results are derived
from the CropWat software for each stages of development, Appendices 1 to 3. Crop
evapotranspiration (Etc) for onion varies from 60 mm when there is no irrigation to 62
mm for both irrigation treatments (current pumping rate and irrigation recommended
by the soft ware) in the initial stage, similarly Etc varies from 95 to 114 mm in the
development stage, 70 to 232 mm in the mid development stage, 29 to 129 mm in the
late stage and 254 to 537 mm for the whole stage of development. Crop
evapotranspiration (Etc) for tomato varies from 60 mm when there is no irrigation to
62 mm for both irrigation treatments (current pumping rate and irrigation
recommended by the soft ware) in the initial stage, similarly Etc varies from 98 to 123
mm in the development stage, 73 to 270 mm in the mid development stage, 32 to 141
mm in the late stage and 263 to 595 mm for the whole stage of development. Crop
evapotranspiration (Etc) for pepper varies from 60 when there is no irrigation to 61
mm for both irrigation treatments (current pumping rate and irrigation recommended
by the soft ware) in the initial stage, and similarly Etc varies from 127 to 151 mm in
the development stage, 74 to 216 mm in the mid development stage, 16 to 122 mm in
the late stage and 274 to 550 mm for the whole stage of development. Net irrigation
that is the amount of water supplied to the field varies among different irrigation
managements and vegetable types. As can be seen from Tables 3 and 4 the net
irrigation amount for the three vegetable during the current pumping rate and
recommended by the soft ware irrigation scenarios are different. The net irrigation for
32
onion during current pumping rate is 645 mm and when it is irrigated by the amount
recommended by the soft ware is 536 mm. The net irrigation for tomato during current
pumping rate is 670 mm and when it is irrigated by the amount recommended by the
soft ware is 594 mm. The net irrigation for pepper during current pumping rate is 670
mm and when it is irrigated by the amount recommended by the soft ware is 550 mm.
irrigation amount recommended by the soft ware is equal to the crop water
requirements of the vegetables, Tables 3 and 4. This shows that the Etc of the crop has
attained its maximum with the recommended irrigation amount. Any further irrigation
could not increase crop evapotranspiration rather it could be lost during irrigation. The
most irrigated treatment gave the maximum ET, and rain-fed had the lowest ET. The
results indicated that the ET of the vegetables was greatly affected by irrigation
application (Sun et al 2006).
The rate of pumping from the ground water in the current condition is 5mm per day
from the start of irrigation to its end. But irrigation amount recommended by the soft
ware varies from stage to stage as the crop water requirements of the crop varies from
stage to stage, Appendices 1 and 2. Irrigation starts on 5 th of March and ends on 12 th
of July for onion, for tomato it starts on 5 th of March and ends on 17 th of July, and
for pepper it starts on 1st of March and ends on 13 th of July.
33
Table 3 : Field water balance under different water management during irrigation for the three vegetables grown in the area. All values are indicted in mm/stage days. See Appendix 8.6. Opti. indicates values when irrigated by the recommended irrigation amounts by the soft ware, Actual indicates values when irrigated by the current pumping rates.
Crop Type Planting date Stage of Development PET CWR Irr. Req. Etc (mm)
Ground water recharge from rainfall and irrigation
In Kobo valley, supplementary irrigation from ground water is supplied from March to
mid July (KGVDP 2006). For the rest of the year, the farm land either produces cereal
in the rainy season or is not cultivated. Comparison of ground water recharge for
periods with irrigation and periods with no irrigation during the year reveal that
recharge will increase from irrigation. Ground water recharge was calculated using the
water balance model (Equation 10). The water balance model was used for two
scenarios to calculate recharge: first, recharge from areal rainfall only (i.e. represent
the situation some 10 years ago when there was no irrigation) and second, recharge
from irrigation and rainfall. Recharge from irrigation indicates recharge to the water
table if the farmlands are irrigated from the ground water at the rate of current
pumping (i.e. 5 mm per day) for all vegetables for about four months from March to
mid July. In the period before irrigation was used, crops were rain-fed and all fields
were fallowed at least every Kiremit; the annual recharge to the aquifer was only from
the areal rainfall. Hence, groundwater recharge was small and steady, pulsing only in
response to intense rainfall during the Kiremit season. Annual and monthly ground
water recharge amounts follow the amount and intensity of annual and monthly
rainfall. Recharge to the ground water is always associated with the condition that,
when the available water over a period is higher than the PET there is recharge. There
is recharge to the water table from rainfall if the amount of rainfall is greater than the
PET over a given period. For Kobo area, there is recharge from the rainfall during
Kiremit season when rainfall is greater than the PET. The annual recharge to the water
table from rainfall is contributed from the recharge during Kiremit season. From
Figure 3, it is clear to see the trend of recharge follows exactly the available water
either from irrigation or precipitation. Except for the year 2001 and 2002, ground
water recharge is directly related to the available water. The recorded annual rainfall
38
in Kobo was 74 cm in 2001 and 61 cm in 2002. The computed annual recharge, by the
T-M model in 2001, is 35 cm during the application of irrigation and 29 cm when
there was no irrigation, and for 2002 the recharge is 13 cm during the application of
irrigation and 26 cm when there is no irrigation. When rainfall decreases from 74cm in
2001 to 61cm in 2002 the recharge increases from 13 cm in 2001 to 26 cm in 2002 for
the scenario where no irrigation water is applied.. The reason is that during 2001 most
of the rainfall events were in the dry months while for the other years the rainfall was
during the wet months. Details are given in Appendix 5.
Figure 3: Mean annual recharge to the ground water when there is irrigation and if there is no irrigation at all. Irrigation is scheduled from March to mid July at the rate of the current pumping. Red line (R pumping) is recharge from irrigation from ground water pumping plus rainfall, and blue line (R RF) is recharge from areal rainfall alone or if there is no irrigation.
Hence, most of the rainfall evaporates during the dry months rather than percolating,
as the PET during the dry months is high. From the results obtained by T-M model,
recharge increases if irrigation is added for crop production, Table 5 and Figure 3. In
the previous section the start and end of irrigation for the three vegetables are
39
indicated. The total amount of rainfall during the growing period of the crops is 19 cm
for onion, 22 cm for tomato and 20 cm for pepper, Table 4 and Appendices 1, 2, 3 and
6.
As Tables 3 and 5 show, under the “actual” pumping (i.e., what is currently used by
farmers and consist of a daily application of 5mm/day), which is about 645 mm per
cropping season for single cropping season, the actual evaporation by the crop (called
crop water requirement in the tables) of the vegetables remain below the net irrigation
applied to the field. Thus more water is added than evaporates and water will
percolate downward. The greater the irrigation rate the more water percolates. This is
shown in Table 5 and Figure 3 where the results obtained by T-M model, recharge
increases if crops are irrigated. As we will see later that does not mean that the
groundwater table increases as well because water is being pumped from the aquifer.
Table 5: Recharge and ground water table (GWTE) during different irrigation (pumping) amount. Actual pumping implies the amount of irrigation water pumped at rate of what farmers are pumping (5mm/day for all vegetables; onion CWR, tomato CWR and Pepper CWR are crop water requirements recommended by the CropWat software for onion, tomato and pepper respectively for one cropping season
Kobo, especially in the research sub-watershed, has a recent history of irrigation.
Irrigation using ground water was started during 2005 in a small part of the study area.
Currently, more bore holes are being drilled to irrigate the whole plain area.
40
Effect of Irrigation with CWR of different crop water requirements on ground
water recharge
With the encouragement of regional and federal government financial and technical
assistance, the quantity of groundwater extracted each year for irrigation has increased
steadily. After irrigation, most crop requirements were met, as indicated by the
leveling off of annual evapotranspiration rates (Tables 3, 5 and 6). Here it is
interesting to see that changing cropping patterns can reverse recharge from the
irrigated field (Table 6). Typical planting and harvesting dates for these vegetables is
assumed as it indicated in the previous section. Ground water recharge is higher if we
plant and irrigate tomato crops according to the crop water requirement (CWR, as
calculated with the CropWat software) than if the same is done for onion and pepper
crops, as the CWR of tomatoes is higher and length of growing season is higher. The
results from Table 6 and Figure 4 indicate that recharge from irrigation to the aquifer
can be minimized if we irrigate crops by their respective crop water requirements.
Different crops have different water requirements so as to get maximum production.
The difference in crop water requirements for different crops results in the difference
in recharge response from different crops, although we can save more water if we
irrigate crops by their crop water requirements (Kendy et al 2004). Figure 4 shows
cumulative model calculated ground water recharge for the three crops.
Table 6: Mean annual recharge to the ground water recharge during single and double cropping season irrigation, the amount of irrigation as the current pumping rate in the area (5 mm/day for the growth period)
Irrigation Rotation Irrigation amount
(cm/year) Recharge (cm/year)
One cropping period 65 39 Two cropping period 114 46
41
The annual changes in groundwater storage were calculated by subtracting inflow
(model-calculated Recharge) from outflow (pumping for irrigation plus base flow to
the nearby river.
Figures 4 and 5 and Tables 3, 4 and 6 show that recharge is not directly related to the
total amount of irrigation applied. Rather, it is a complex relationship of daily rainfall
and irrigation. For the years from 1998 to 2004, recharge was greater under irrigation
with tomato CWR (i.e., irrigation amounts calculated with the CropWat software)
although the total irrigation was less than the actual pumping rate. This may be
because the CWR of tomato during the mid development stage of the crop is higher
than the actual pumping rate.
Figure 4: Mean annual recharge to the ground water if irrigation was started in 1997, and recharge to the ground water when it is irrigated with the current pumping rate (5mm/day for one cropping season) R pumping, and according to CropWat software calculated onion crop water requirement R Onion CWR, tomato crop water requirement, R Tomato CWR and pepper water requirement R Pepper CWR. the amount shown is the total recharge as a result of both rainfall and irrigation.
42
Figure 5: Mean annual recharge to the ground water when irrigation has been stared in 2005 to 2007 as the actual condition in the area and recharge to the ground water when it is irrigated with the current pumping amount R pumping, and according to CropWat software calculated onion crop water requirement R Onion CWR, tomato crop water requirement, R Tomato CWR and pepper water requirement R Pepper CWR.
Comparison of ground water recharge during irrigation period and periods without
irrigation in a year reveal that recharge will increase from irrigation. Figure 6 shows
that irrigation will increase recharge to 6 cm, while it was less than 2cm if irrigation
were not applied from March to May. It is also possible to see the lag effect of
irrigation to the ground water recharge. Although irrigation was ceased at the
beginning of July, the recharge of July and the first week of August was shifted
upwards from the base case, i.e. if irrigation were not supplied.
43
Figure 6: Average monthly recharge to the ground water for the two scenarios; if there was irrigation since 1997 and if there was no irrigation to the present i.e. 2008. Irrigation refers to the amount of water pumped out at the rate of what farmers are using for single growing season (5mm/day for the whole growing season)
Future Irrigation scenario’s and ground water recharge
Water requirements of crops are met, in part, by rainfall, contribution of moisture from
the soil profile and applied irrigation water. A part of the water applied to irrigated
fields for growing crops is lost in consumptive use and the balance infiltrates as
recharge to the ground water. The amount, period, and time of irrigation application to
the agricultural field could be used to estimate the annual recharge to the ground
water. In this study, irrigation is applied from March to mid July during one irrigation
season for the three vegetables from 2005 to 2007. Moreover, these vegetables can be
cultivated two times a year in addition to cereal production during the main rainy
period (August to October).
Therefore, another irrigation period is proposed to be from mid November to mid
March. This will not affect the cultivation of cereals in the main rainy season. As
irrigation time increases, the amount of water delivered also increases. In this case,
analysis is done in such a way that irrigation is applied from March to Mid July for the
44
single irrigation season and from November to mid March and from April to mid July
for the double irrigation season. Table 6 shows when the irrigation depth, associated
with increase of irrigation period, increases from 65 to 114 cm and the annual recharge
to the ground water increases from 39 to 46 cm. This implies that, if we irrigate for
longer time during a year, the recharge will be increased. This shows that
indiscriminate use of irrigation water, has led to problems of rising water tables
causing widespread land degradation (Schofield et al., 1989; Anderson et al., 1993).
The irrigation pattern of this area was single up to 2007. But starting from 2008, the
government gave more emphasis to produce vegetables two times during the moisture
deficit periods of the year. Hence, ground water recharge estimation for the coming 10
years was calculated. Figure 7 shows the annual recharge if irrigation is practiced once
a year or twice a year. The T-M water balance model indicates that if irrigation
duration increases recharge will also increase.
Figure 7: Mean annual recharge to the ground water when irrigation has been stared in 2005 and continue to 2018 as the current pumping rate for one cropping season and (Blue line) and irrigation duration increased for two cropping season from 2008 to 2018 at the rate of current pumping rate (Red line).
45
Ground water table depth
Ground water table depth with time was estimated using the T-M equation, a simple
water balance formula that balances inflow and outflow of the water in an area
(Equation 15-18). Darcy’s law (Equation 14) was also used to estimate base flow to
the river from the agricultural fields, since ground water table depth was higher than
the river bed. Figure 8 and Table 5 show the ground water table depth in different
irrigation management scenarios. Recharge and change in storage are related linearly
for areas irrigated from river water, but for ground water irrigation the relationship
among recharge, irrigation, change in storage and ground water depth is complex. It is
a complex relationship of irrigation, rainfall and evapotranspiration (Kendy et al.,
2002).
Figure 8: Ground water table elevation from the well surface if irrigation was started in 1997, GWTE Pumping, GWTE onion CWR, GWTE Tomato CWR, and GWTE pepper CWR indicating water table elevation during pumping with actual condition, onion crop water requirement, tomato crop water requirement and pepper crop water requirement respectively. Negative sign indicates depth from the surface
46
The difference in crop water requirements for different crops results in the difference
of recharge response from different crops (Kendy et al 2004) and hence, the rate of
ground water decline too. Figure 8 shows the estimated water table depth change due
to different agricultural water use in Kobo. Ground water table will decline more for
the current pumping practices than pumping the three crop water requirements
recommended by the software is small. This is because, fields are irrigated with the
crop water requirements of each vegetables and the difference in crop water
requirements among these vegetables is also small. But the difference in the effect of
ground water decline between the current pumping rate and pumping recommended by
the software is high. This may be because of the difference in the irrigation amount
and the difference between potential evapotranspiration and crop evapotranspiration of
these vegetables is high. Crops transpire at their crop evapotranspiration rate if the
available water is equal to the crop water requirement of the particular crop, but if
there is much water above the CWR, more water will transpire and the ground water
table will decline at a higher rate. On average, ground water table declines by 53 cm
for current pumping rate, 0.28 cm for onion CWR, 0.31 cm for tomato CWR and 0.31
cm for pepper CWR per year (Tables 3, 4, 5 and 6). This implies that the more the
ground water is pumped out, the higher the rate of decline, unlike recharge to the
ground water. Although the rate of water table decline in pepper CWR should be less
than that of tomato, it has higher value. The reason may be due to the longer growing
period of pepper than tomato. This could affect the daily water balance and water table
status of the area. The crop requirements, however, remained steady, as indicated by
evapotranspiration rates, and the excess water percolated through fields and recharged
aquifers at an accelerated rate. Irrigation with the CWR of each crop will decrease at
about the same rate as pumping, so the net groundwater withdrawals
47
(evapotranspiration) remained relatively constant. Consequently, groundwater levels
continued to decline, despite reduced pumping due to increased application efficiency.
Figure 9 shows water table depths when irrigation was started in 2005. It is assumed
that the ground water table was 18m below the surface of the well during the start of
operation. As from the report of well log and pumping tests done by the KGVDP, the
average static water level of the wells is 18m. Measurement using GPS indicated that
the bed floor of Hormat River is about 40m (vertical distance) from the surface of the
well and 200 m away from the well (horizontal distance). Water flows out from the
farmland to the river following the slope. This is one indication of ground water
recharge to the river. Since water table depth is higher than the river bed water will
flow to the river as a base flow. As it is clearly seen in the well logo report, water
entering the area will either recharge the ground water table or discharge to the river as
subsurface flow.
From Figure 9, water table level declined by 1.5m during the three year irrigation time
of four months of irrigation per year. But if onion, tomato or pepper were cultivated
and irrigated with the respective CWR, the water table level would decline by 86 cm
for onion CWR, 87 cm for tomato CWR and 91 cm for pepper CWR during the three
year irrigation time of four months irrigation per year. This shows onion contributes
least to the decrease of water table level during pumping with its CWR.
48
Figure 9: Ground water table elevation from the well surface if irrigation was started in 2005. GWTE Pumping, GWTE onion CWR, GWTE Tomato CWR, and GWTE pepper CWR indicating water table elevation during pumping with actual condition, onion crop water requirement, tomato crop water requirement and pepper crop water requirement respectively.
Effect of irrigation area on the ground water depth
Maximum crop production could be achieved if all agricultural land is used through
intensive irrigation and crop management activities. Irrigation application to the total
irrigable land could affect the plain ground and sub surface water balance. Figure 10
shows the effect of irrigating different proportions of agricultural land if irrigation had
been started in 1997. Table 7 shows the average annual change in storage for different
irrigated farm land if irrigation had been started in 1997.
Table 7: Average annual change in storage from different irrigation area if irrigation has been started in 1997
Area proportion Airr/Atotal=0.25 Airr/Atotal=0.5 Airr/Atotal=0.75 Airr/Atotal=1
Change in storage per year (m/year) 0.00 -0.04 -0.16 -0.29
Change in water table per year
(m/year) 0.00 0.13 0.53 0.95
2007 water table depth (m) -18.0 -19.4 -23.8 -28.5
49
Figure 10: Ground water table elevation from the well surface if irrigation was started in 1997 for different irrigated to irrigable land area ratios. A irr and A total denotes irrigated and total irrigable land
As indicated by Table 7, change in ground water storage varies significantly for
different irrigation areas. Ground water balances using different irrigated areas reveals
that irrigating smaller areas of the total irrigable land will not have an effect on the
annual ground water table. Irrigating 25% of the total irrigable land will not have an
impact on the ground water table as far as ground water irrigation is concerned. But, if
the area of irrigation increased to 50%, 75% and 100%, water table will be affected.
The water table will be affected more (more sensitive) for irrigation areas of more than
75% of the total area. If irrigation had been started in 1997 with all irrigable field
irrigated at the rate of current actual pumping, the water table would have declined to
28.5 m from the surface. This indicates that the water table would decline by 10.5 m in
11 years of irrigation. Hence, the ground water table declines by 13 cm for 50%, 53
cm for 75% and 95 cm for 100% of total irrigated area. However, the water table will
not be affected if the total irrigated field is below 25% of the total irrigable area.
50
Although irrigating all areas for crop production is key factor for maximizing crop
production and ensuring food security, ground water table is affected.
Table 8 and Figure 11 show the average change in ground water storage and annual
water table depth for different irrigable areas if irrigation had been started in 2005 for
single cropping irrigation patterns. This reflects the current scenario implemented by
the project in the area. Irrigation was started in 2005 for some wells as a testing and
awareness creation for the farmers. Now more bore holes are used for irrigation. There
are about 39 bore hole ready for irrigation starting from 2009 (well completion report,
2008). Hence, monitoring ground water table and different components of ground
water balance is important.
Table 8: Average annual change in storage from different irrigation area if irrigation were started in 2005
Area proportion Airr/Atotal=0.25 Airr/Atotal=0.5 Airr/Atotal=0.75 Airr/Atotal=1 Change in storage per year (m/year) 0.00 0.04 0.15 0.27 Change in water table per year (m/year) 0.00 0.13 0.49 0.91 2007 water table depth (m) -18.0 -18.4 -19.5 -20.7
As seen in Table 8, ground water table levels will decline by 13 cm if 50%, 49 cm if
75% and 91 cm if 100% of irrigable land is irrigated in the 3 year irrigation history of
four months of irrigation in a year. Measurement of the water table depth in December
2008 on Hormat-Golina No.4 well indicated that the area irrigated is only 75% of the
total area irrigable by the well. It decreased by about 50cm from the previous year.
Water table depth at the well testing time was 17.5 m, and after one year it declined to
17.05 m.
51
Figure 11 : Ground water table elevation from the well surface if irrigation was started in 2005 for different irrigated to irrigable land area ratios. A irr and A total denotes irrigated and total irrigable land
Therefore, Tables 8 and 9, and Figures 10 and 11 indicate that ground water table level
and change in ground water storage are sensitive to the area of farm land supplied by a
ground water irrigation system. Best management practices that balance recharge,
pumping and loss according to the crop water requirements of crops and crop rotation
will decrease the decline of the water table level.
Ground water depth in the future
Future ground water depth scenarios are developed by taking the rainfall in the
previous 11 years as data to simulate the coming 11 years. This will have limitations
since rainfall events change from year to year. But as annual ground water table and
recharge from irrigation and rainfall will not vary significantly, rainfall data is
extrapolated for this analysis. Future ground water table level predictions include the
following scenarios: irrigation is used for a single cropping season at the rate of the
current actual draft for different irrigated land areas, irrigation is used for two
52
cropping seasons in the moisture deficit periods for different areas of irrigated land
and irrigation is used by CWR of the three vegetables.
Water table status under single and double cropping irrigation
The cropping pattern of the area affects the amount of irrigation applied on the field.
Long growing crops need more water than short growing crops, assuming the crop
water requirements of both crops in all stages of development are similar. Irrigating an
area more than once a year affects the water balance of the area as more water is used.
Double cropping with irrigation needs much more water than single cropping. This
scenario assesses the fate of the water table level if the current pumping rate is used
for irrigating farm land. Ground water table and recharge was analysed for two
cropping seasons and single cropping season scenarios. A water balance using T-M
equation (Equation 12-18) gave that, if the irrigation period is increased from one
cropping season to double cropping, the water table level will decline at a faster rate.
For the two scenarios, irrigation will start from March to mid July for the single
irrigation season and from November to mid March and from April to mid July for
double irrigation season. The modelled cropping pattern is based on the assumption
that short growing crop varieties could be cultivated three times a year. It is also
assumed that the area could be cultivated twice a year starting from 2008 onwards
during dry months of the year. Hence, the water table level after 11 years will be 31.2
m below the surface for double cropping irrigation and 25.2 m below the surface for
single cropping irrigation. This implies that water table level will decline by 6 m more
for two cropping season irrigation than single cropping season irrigation in 11 years of
irrigation.
53
Figure 12 shows the water table depth for single and double season irrigation. The
water table will decline by 52 cm per year for a single cropping season and 94 cm per
year for a double cropping system if the pumping rate continues as is for the future 11
years. Here, the cropping period for a single cropping season is from March to the first
week of July, and, for a double cropping system, it starts from mid November to mid
February and from mid March to the end of June. For the double cropping season
irrigation duration during each season, the overall time is reduced so as to have
enough time for cereal cultivation in the rain period. Here, short growing varieties are
recommended in order to produce three times a year.
Figure 12: Ground water table elevation from the well surface. GWTE Twice Pumping, is water table elevation if irrigate for two cropping seasons in a year from 2008 to 2018 and GWTE single Pumping is water table elevation if irrigate for one cropping season in a year from 2008 to 2018
As it can be seen from Table 9 and Figure 12, the total rainfall for single and double
cropping season systems is almost equal, but the PET during the seven months
irrigation period is higher than the four months irrigation period. The water table level
declines more for the double cropping irrigation system because the PET during these
months is higher than the PET during the months of the single cropping season,
54
although the value of rainfall is similar. This indicates that ground water table status is
not only the function of available water; rather, it is a complex function of PET,
rainfall and other parameters.
Table 9: Ground water recharge, change in storage and water table height for single and double irrigation scenarios. Single denote for irrigation period from March to mid July and for double irrigation duration the first irrigation is from mid November to mid February and the second irrigation is from mid March to end of June.
Water table status under single and double cropping irrigation for different area
of irrigated field
As is seen in the previous section, water table depth for different irrigated areas affects
the ground and subsurface water balance and hence, the depth and annual recharge of
ground water from irrigated fields. Future scenarios for the coming 11 years have been
developed for different irrigation areas and cropping patterns associated with
irrigation.
Table 10 shows ground water table depth for different irrigated areas under single and
double cropping season irrigation if irrigation was started in 2005 with single cropping
and 2008 for double cropping season irrigation. Change in storage and water table
level will not be affected in the future for irrigation areas with a ratio of irrigated to
total irrigable land of up to 0.5. Analysis of the recharge and water table level using
the T-M equation and ground water balance for the current and future gave that the
water table level will decline at a higher rate for a double cropping season irrigation
55
system if all the available area is irrigated. As seen from Figure 13 and Table 10, the
ground water table level during a single cropping season will drop to 25.2 m and 31.1
m below the surface for 75% and 100% irrigated area, respectively. This shows that
the maximum ground water depth reduction for a single cropping irrigation system is
12 m in 14 years of irrigation period starting from 2005 and running to 2018.
Table 10: Average annual change in storage and water table depth after 11 years in the future (2018) for different irrigation area under single and double cropping season irrigation. Double irrigation starts in 2008 to 2018
Figure 13: Ground water table depth from the well surface for one irrigation period in a year under different irrigated to irrigable land area ratios.
56
Figure 14 shows the water table depth for different irrigated area proportions for a
single cropping season irrigation system from 2005 to 2007 and a double cropping
season irrigation system from 2008 to 2018. Irrigation area under the double cropping
season system will result in a greater reduction of the ground water table level. From
Figure 14 and Table 10, it can be observed that the water table level will drop below
the surface by 23.4 m for 50%, 31.2 m for 75% and 39.1 m for 100% of irrigated field
usage for a double cropping season irrigation system.
Sensitivity to the ratio irrigation area is greater for the double cropping irrigation
system than for the single cropping season system as seen from Table 10 and Figures
13 and 14. The water table level will decline by 6 m more if the irrigation area ratio is
increased from 75% to 100% for the single cropping season irrigation system and by 8
m for the double cropping season irrigation system. If the whole plain area could be
irrigated twice a year from 2008 on, the water table level at the end of 2018 will be
39.1 m below the surface of the well, i.e. the water table will drop by 21 m in11years
rate of 2 m/year.
Figure 14: Ground water table depth from the well surface for two irrigation period in a year from 2008 to 2018 under different irrigated to irrigable land area ratios
57
Water table status under single and double cropping irrigation for different
CWR pumping
As irrigation depth and ground water pumping has an effect on ground water recharge
and water table depth (Kendy et al 2004), irrigation with the crop water requirements
of vegetables will have different effects on the water table depth for the future. Figure
15 shows the response of the water table depth to different depths of irrigation if
irrigation had been started in 2005 for single cropping season irrigation up to 2018. As
the current scenario, a single cropping irrigation system for the onion crop has less of
an effect on the water table level. A one season cultivation of onion will result in a
final water table depth of 21.9 m below the surface of the well where as tomato and
pepper will have 22.2m and 22.3 m level, respectively. But if the current actual
pumping continues, the water table level will decline to 25.2 m below the surface, as
can be seen in Table 11.
Figure 15: Ground water table depth from 1997 to 2018 if irrigation started in 2005 to 2007 single and continue similarly up to 2018
58
This implies that the difference in the effect of the three vegetable crops if irrigated
with their CWR is insignificant as compared with the current pumping rate. Although
every pumping rate has resulted in the decline of water table level, irrigating with the
crop water requirement of crops being grown will protect the water table from further
reduction.
Assessment of the water table depth for the future scenario calculation if the three
vegetable crops are to be cultivated two times a year from 2008 to 2018 is shown in
Table 11 and Figure 16. Results of the ground water and soil-water balance using the
T-M model and a simple water balance that balances inflow and outflow in the ground
water system indicate that there is insignificant difference between the three vegetable
crops. The changes in storage for the four irrigation scenarios are almost similar, as
can be seen in Table 11.
Table 11: Average annual change in storage and water table depth after 11 years in the future (2018) if the area is irrigated by the current pumping rate, onion crop water requirement, tomato crop water requirement and pepper crop water requirement under single and double cropping season irrigation. Double irrigation starts in 2008 to 2018.
Cropping type/pattern Pumping
Average annual change in storage
(m/year)
Change in ground water table (m/year)
2018 water table depth
(m)
Single cropping season irrigation
Actual -0.16 0.52 -25.2 Onion CWR -0.085 0.28 -21.9
Tomato CWR -0.090 0.3
-22.2
Pepper CWR -0.093 0.3 -22.3
Double cropping season irrigation
Actual -0.306 0.94 -31.2 Onion CWR -0.312 0.84 -30.6
Tomato CWR -0.334 0.95
-31.5
Pepper CWR -0.317 0.92 -30.8
59
Figure 16: GWTE from 1997 to 2018 if irrigation started in 2005 to 2007 single and twice a year from 2008 to 2018
The comparison of annual change in ground water storage between single cropping
and double cropping irrigation systems show that the negative change storage will
increase by almost a factor of two in the double cropping season system. If the
cropping pattern of the area continues as the current practice, the maximum water
table depth will be 25.2 m below the surface of the well at the end 2018. Therefore,
water table depth will decrease by 7.2 m from the beginning of the irrigation period as
seen from Table 11 and Figure 15. However, if the cropping pattern is changed to two
irrigated cropping seasons from 2008, the water table level will decrease to 31.5 m
below the surface.
As shown in Table 11 and Figure 16, the water table depth will decline by 13.5 m
during a three year single irrigation cropping season system plus a 14 year double
irrigation cropping seasons system scenario. This implies that rate of water table depth
reduction and cropping seasons are linearly related. If irrigation duration doubles
following cropping pattern, the water table level will decline by factor of two.
60
The effect of irrigation scheduling on the ground water recharge and water table
decline is different for different irrigation duration than the depth of irrigation. As can
be seen from Figure 15 and Table 11, water table declines at different rate following
the depth of irrigation, if we irrigate the area for fewer months in a year. But Figure 16
and Table 11 shows that the rate of water table decline does not vary significantly if
we irrigate for more months in a year.
61
CHAPTER SIX 6. Conclusions and Recommendations Conclusions
In areas where ground water is used for irrigation, as the Kobo valley, groundwater
modeling is an important tool for quantifying the groundwater balance which is an
essential prerequisite for sound, scientific groundwater management. The
Thornthwaite Mather equation and a simple soil water balance formula could be used
to quantify the areal recharge due to irrigation and rainfall. By generating an
independent estimate of areal recharge, the soil-water balance model presented in this
paper also provides an important constraint on estimates of lateral recharge needed for
groundwater modeling.
Sustainable use of ground water in arid and semi arid areas could be achieved if
farmers are irrigating their farm lands with the CWR of the crops to be grown. Ground
water table levels will continue to decline if pumping continues; however, the rate of
decline could be decreased to the allowable level by following different agronomic
practices, which could increase recharge to the water table and decrease the rate of
evaporation. As the livelihood of the farmers in the area is in the worst condition,
pumping, regardless of recharge and water table depth is allowable for the coming two
decades. The maximum water table reduction rate is about 2 m per year for the coming
10 years.
Recommendations
The results obtained from in this are similar to the study by Kendy et al. (2004) for the
North China Plain. Increased acreage of irrigation decreased the ground water table.
This resulted in increased pumping costs over time and at the same time a decrease in
62
base flow. If the entire Kobo Valley was irrigated, the ground water could decrease
by as much as 2 m per year. Estimates of ground water decline should be refined by
by a more realistic simulation of the water recharged from the river into aquifers once
the ground water has declined below the river channel.
In order to assess the decline of ground water, monitoring wells should be installed and
ground water table monitored monthly. Stream gauging stations should be established
as well. This will allow validation of the simulation model and the assessment of the
interaction of the surface ground water system.
63
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68
APPENDICES 8.1 Crop water requirements of different vegetables during one day interval irrigation scheduling as recommended by the CropWat soft ware. 8.1.1 Onion
CropWat 4 Windows Ver 4.3 Crop Water Requirements Report
* ETo data is distributed using polynomial curve fitting. * Rainfall data is distributed using polynomial curve fitting.
72
8.1.2 Tomato
Crop Water Requirements Report Crop Name #: Tomato Block #: [All blocks] Planting date :5-Mar Calculation time step =1 Day(s) Irrigation Efficiency =90% * ETo data is distributed using polynomial curve fitting. * Rainfall data is distributed using polynomial curve fitting.
Estimated yield reduction in growth stage # 1 = 0 % Estimated yield reduction in growth stage # 2 = 0 % Estimated yield reduction in growth stage # 3 = 0 % Estimated yield reduction in growth stage # 14= 0 % Estimated total yield reduction =0% * These estimates may be used as guidelines and not as actual figures. * Legend: TAM = Total Available Moisture = (FC%- WP %)* Root Depth [mm]. RAM = Readily Available Moisture = TAM* P [mm]. SMD = Soil Moisture Deficit [mm].
88
8.2.3 Pepper
CropWat4 Windows Ver 4.3 Irrigation Scheduling Report