Losses

Losses

Interception, Evaporation, Transpiration, Evapo-transpiration, Infiltration

Precipitation – losses = Run off

Definitions Interception

• Interception is defined as the precipitation which is caught by vegetation, buildings and other subjects and never reaches the land surface

Evaporation• Evaporation is defined as the process by which water

is changed from liquid or solid state into the gaseous state through the transfer of heat energy.

Transpiration• Transpiration is the process where by the moisture

that has circulated through the plant structure is returned to the atmosphere

Evapotranspiration• Evapotranspiration is evaporation and transpiration

combined, including evaporation from lakes. It is the evaporation from all water, soil, snow, ice, vegetative, and other plus transpiration. It is some time called as consumptive use of water.

Interception

The amount of precipitation intercepted by plants varies with leaf type, canopy architecture, wind speed, available radiation, temperature, and the humidity of the atmosphere. • It is normally about 20 to 30 % of

the gross rainfall depending on type of vegetation.

Interception can be technically defined as the capture of precipitation by the plant canopy and its subsequent return to the atmosphere through evaporation or sublimation.

Interception (cont..) Interception is not important in

heavy floods and heavy rainfall. But important for small storms as it

would not reach ground and much water stored goes back as evaporation.

It is important especially in forest areas and it varies with different type of vegetation.

Controls on Interception Lossesand Throughfall

• Storms size and frequency• Hardwoods vs conifers• Growing vs dormant seasons• Snow vs rain• Stand density• Energy availability• Position under canopy

Rainfall Interception

Interception: Ic = Pg – Th – Sf

Frontal Rainfall: Events (Feb. 22-24 2002), Avalon, FL

Hours After Rainfall Start0 5 10 15 20 25 30 35 40

Rai

nfal

l Int

ensi

ty

-0.05

0.00

0.05

0.10

0.15Rain-RM

Inside

Interception

Cum

ulat

ive

Rai

nfal

l (in

)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Total Rain-Outsite Citrus CanopyTotal Rain-Inside Citrus CanopyTotal Canopy Interception

Rainfall Canopy Interception

EVAPORATION PROCESSEvaporation is the process in which a liquid changes to the gaseous state at the free surface, below the boiling point through the transfer of heat energy.

The rate of evaporation is dependent on         • Vapour pressure at the water surface         • Air and water temperatures,         • Wind speed,         • Atmospheric pressure,         • Quality of water and         • Size of the water body.

Evaporation

Evaporation (CONT..) Vapour Pressure

The rate of evaporation is proportional to the difference between the saturation vapour pressure at the water temperature, ew and the actural vapour pressure in the air, ea.

Thus EL = C (ew -ea)                               

• Where, EL = rate of evaporation (mm/day) and C = a constant; ew and ea, are in mm of mercury. This Equation is known as Dalton's law of evaporation after John Dalton (1802) who first recognised this law. Evaporation continue till ew = ea. If ew > ea, condensation takes place.

Evaporation (CONT..) Temperature

• Other factors remaining same, the rate of evaporation increases with an increase in the water temperature.

Wind

• Wind aids in removing the evaporated water vapour from the zone of evaporation and consequently creates greater scope for evaporation

Atmospheric Pressure• Other factors remaining same, a decrease in the barometric

pressure, as in high altitudes, increases evaporation Soluble Salts

• When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence causes reduction in the rate of evaporation. The per cent reduction in evaporation approximately corresponds to the percentage increase in the specific gravity. Thus, for example, under identical conditions evaporation from sea water is about 2-3% less than that from fresh water

Estimation of evaporation The amount of water evaporated from a

water surface is estimated by the following methods:

• Evaporimeters,     • Empirical evaporation

equations / Analytical methods.

Types of Evaporimeters Evaporimeters are water-containing pans

which are exposed to the atmosphere and the loss of water by evaporation measured in them at regular intervals.

Many types of evaporimeters are in use and a few commonly used pans are :

• Class A Evaporation Pan• ISI Standard Pan• Colorado Sunken Pan• US Geological Survey Floating Pan

Class A Evaporation Pan Class A Evaporation Pan

• It is a standard pan of 1210 mm diameter and 255 mm depth used by the US Weather Bureau and is known as Class A Land Pan.

The depth of water is maintained between 18 cm and 20 cm (Fig. 3.1). The pan is normally made of unpainted galvanised iron sheet. The pan is placed on a wooden platform of 15 cm height above the ground to allow free circulation of air below the pan. Evaporation measurements are made by measuring the depth of water with a hook gauge in a stilling well.

ISI Standard Pan

The pan is made of copper sheet of 0.9 mm thickness, tinned inside and painted white outside (Fig. 3.2).

A fixed point gauge indicates the level of water. A calibrated cylindrical measure is used to add or remove water maintaining the water level in the pan to a fixed mark.

The top of the pan is covered fully with a hexagonal wire netting of galvanized iron to protect the water in the pan from birds.

Further, the presence of a wire mesh makes the water temperature more uniform during day and night.

The evaporation from this pan is found to be less by about 14% compared to that from unscreened pan.

The pan is placed over a square wooden platform of 1225 mm width and 100 mm height to enable circulation of air underneath the pan.

This pan evaporimeter specified by IS:5973-1970, also known as modified Class A Pan, consists of a pan 1220 mm in diameter with 255 mm of depth.

Colorado Sunken Pan

The chief advantage of the sunken pan is that radiation and aerodynamic characteristics are similar to those of a lake.

However, it has the disadvantages like difficult to detect leaks, extra care is needed to keep the surrounding area free from tall grass, dust etc. and expensive to install.

Colorado Sunken Pan

This pan, 920 mm square and 460 mm deep is made up of unpainted galvanised iron sheet and buried into the ground within 100 mm of the top (Fig. 3.3).

US Geological Survey Floating Pan

US Geological Survey Floating PanWith a view to simulate the characteristics of a large body of water, this square pan (900 mm side and 450 mm depth) supported by drum floats in the middle of a raft (4.25 m X 4.87 m) is set afloat in a lake.

The water level in the pan is kept at the same level as the lake leaving a rim of 75 mm.

Diagonal baffles provided in the pan reduce the surging in the pan due to wave action.

Its high cost of installation and maintenance together with the difficulty involved in performing measurements are its main disadvantages.

Pan Coefficient, Cp ETo = Cp X pan evaporation in which Cp = pan coefficient. The values of Cp in use

for different pan are given in Table 3. I. ETo= Reference evapotranspiration/(lake Evaporation)

Evaporation Stations It is usual to install evaporation pans in such

locations where other meteorological data are also simultaneously collected.

Currently India has about 200 pan-evaporimeter stations maintained by the India Meteorological Department

The WMO recommend the minimum network of evaporimeter stations as below:        • Arid zones-One station for every 30,000 km²,      • Humid temperate climates-one station for every 50,000 km², and      • Cold regions-One station for every 100,000 km².

Penman equation

where:   E evaporation [mm]  g Bowen constant [kPa/°C]

 D slope of the maximum curve tension of the saturated air with vapours

depending on temperature  l vapourization constant heat at constant pressure, (= 2.45 [MJ/kg])  e report vapours molecule weight /air sec, (= 0.622)  P atmospheric pressure [kPa]

 C

pvapourization constant heat at constant pressure, Cp = 1.013×10-3 [MJ/kg/°C]

 E

aevaporation calculated with Rohwer formula [mm]

 E

cevaporation measured with Colorado bank [mm]

PENMAN-MONTEITH Equation

where:

 R

nnet solar radiation [W/m2]

  g Bowen constant [kPa/°C]

 D slope of the maximum curve tension of the saturated air with vapours depending on

temperature

  l vapourization constant heat at constant pressure, (= 2.45 [MJ/kg])

  r air volumic mass [kg/m3]

 dehumidity deficit [kPa]

 c

pmoist air heat capacity [MJ/kg/°C]

 r

aaerodynamic resistance [s/m]

 r

sdiffusion resistance of evaporation surface [s/ m]

The Blaney-Criddle Method ETo = p*(0.46*Tmean + 8) where:

• ETo = reference crop evapotranspiration (mm/day)Tmean = mean daily temperature (° C)p = mean daily percentage of annual daytime hours.

The Blaney-Criddle Method always refers to mean monthly values, both for the temperature and the ETo.

The Blaney-Criddle Method

p = mean daily percentage of annual daytime hours

If in a local meteorological station the daily minimum and maximum temperatures are measured, the mean daily temperature is calculated as follows: .

MEAN DAILY PERCENTAGE (p) OF ANNUAL DAYTIME HOURS FOR

DIFFERENT LATITUDES

Latitude:

North Jan Feb Mar Apr May Jun July Aug Sept Oct Nov DecSouth July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June60° .15 .20 .26 .32 .38 .41 .40 .34 .28 .22 .17 .1355 .17 .21 .26 .32 .36 .39 .38 .33 .28 .23 .18 .1650 .19 .23 .27 .31 .34 .36 .35 .32 .28 .24 .20 .1845 .20 .23 .27 .30 .34 .35 .34 .32 .28 .24 .21 .2040 .22 .24 .27 .30 .32 .34 .33 .31 .28 .25 .22 .2135 .23 .25 .27 .29 .31 .32 .32 .30 .28 .25 .23 .2230 .24 .25 .27 .29 .31 .32 .31 .30 .28 .26 .24 .2325 .24 .26 .27 .29 .30 .31 .31 .29 .28 .26 .25 .2420 .25 .26 .27 .28 .29 .30 .30 .29 .28 .26 .25 .2515 .26 .26 .27 .28 .29 .29 .29 .28 .28 .27 .26 .2510 .26 .27 .27 .28 .28 .29 .29 .28 .28 .27 .26 .265 .27 .27 .27 .28 .28 .28 .28 .28 .28 .27 .27 .270 .27 .27 .27 .27 .27 .27 .27 .27 .27 .27 .27 .27

Weather station AHMADABAD is at about 23.07°N 72.59°E. Height about 55m / 180 feet above sea level.24-hr Average Temperature

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year°C 20.3 22.8 27.5 31.3 33.8 32.8 29.5 28.3 28.8 28.6 24.8 21.6 27.5°F 68.5 73 81.5 88.3 92.8 91 85.1 82.9 83.8 83.5 76.6 70.9 81.5Source: AHMADABAD data derived from GHCN 1. 596 months between 1941 and 1990

ETo = p*(0.46*Tmean + 8)

=0.306*((0.46*29.5)+8) 6.60 mm/ day (Mean Monthly value

Climatic zone Mean daily temperature

15° 15-25°C 25°

Desert/arid 4-6 7-8 9-10

Semi-arid 4-5 6-7 8-9

Sub-humid 3-4 5-6 7-8

Humid 1-2 3-4 5-6

INDICATIVE VALUES OF Eto (mm/day)

TRANSPIRATION

The important factors affecting transpiration are: • Atmospheric vapour pressure, temperature,

wind, light intensity and characteristics of the plant, such as the root and leaf systems.

Major difference exists between transpiration and evaporation.

Transpiration is essentially confined to daylight hours and the rate of transpiration depends upon the growth periods of the plant.

Evaporation, on the other hand, continues all through the day and night although the rates are different.

• Transpiration is the process by which water leaves the body of a living plant and reach the atmosphere as water vapour.

• The water is taken up by the plant-root system and escapes through the leaves.

EVAPOTRANSPIRATION

While transpiration takes place, the land area in which plants stand also lose moisture by the evaporation of water from soil and water bodies. As such, evaporation and transpiration processes can be considered advantageously under one head as evapotranspiration.

The term consumptive use is also used to denote this loss by evapotranspiration.

For a given set of atmospheric conditions, evapotranspiration obviously depends on the availability of water. If sufficient moisture is always available to completely meet the needs of vegetation fully covering the area, the resulting evapotranspiration is called potential evapotranspiration (PET). Potential evapotranspiration not only depends on soil and plant factors but depends essentially on climatic factors.

Potential Evapotranspiration

The real evapotranspiration occurring in a specific situation is called actual evapotranspiration (AET).

where: ET evapotranspiration [unit of height] or [unit of volume / unit of time]Ew evaporation from the water surface [unit of height] or [unit of volume / unit of

time]Et transpiration produced by the vegetation biological process [unit of height] or [unit

of volume / unit of time]Es evaporation from soil surface with lack of vegetation [unit of height] or [unit of

volume / unit of time]Ei evaporation of rainfall quantity intercepted by the vegetal covering and also by the

constructions [unit of height] or [unit of volume / unit of time]Ed evaporation of rainfall quantity accumulated in ground depressions and without

possibilities of infiltration [unit of height] or [unit of volume / unit of time]Eg evaporation from the snow and glacier surface [unit of height] or [unit of volume /

unit of time]

MEASUREMENT OF EVAPOTRANSPIRATION The measurement of evapotranspiration for a

given vegetation type can be carried out in two ways: either by using • lysimeters • field plots.

Lysimeters• A lysimeter is a special watertight tank containing a

block of soil and set in a field of growing plants. The plants grown in the lysimeter are the same as in the surrounding field. Evapotranspiration is estimated in terms of the amount of water required to maintain constant moisture conditions within the tank measured either volumetrically or gravimetrically through an arrangement made in the lysimeter.

Lysimeters should be designed to accurately reproduce the soil conditions, moisture content, type and size of the vegetation of the surrounding area. They should be so burried that the soil is at the same level inside and outside the container. Lysimeter studies are time-consuming and expensive

Field Plots

In special plots all the elements of the water budget in a known interval of time are measured and the evapotranspiration determined

Evapotranspiration = [precipitation + irrigation input - runoff - increase in soil storage                                   - groundwater loss]

Measurements are usually confined to precipitation, irrigation input, surface runoff and soil moisture. Groundwater loss due to deep percolation is difficult to measure and can be minimised by keeping the moisture condition of the plot at the field capacity. This method provides fairly reliable results.

POTENTIAL EVAPOTRANSPIRATION OVER

INDIA POTENTIAL EVAPOTRANSPIRATION

OVER INDIAUsing Penman's equation and the available climatalogical data, PET estimated for the country has been made. It is seen that the annual PET ranges from 140 to 180 cm over most parts of the country.

The annual PET is highest at Rajkot, Gujarat with a value or 214.5 cm. Extreme south-east of Tamil Nadu also show high average values greater than 180 cm.

The highest PET for southern peninsula is at Tiruchirapalli, Tamil Nadu with a value of 209 cm.

INFILTRATION PROCESS

Infiltration plays a very significant role in the runoff process by affecting the timing, distribution and magnitude of the surface runoff. Further, infiltration is the primary step in the natural groundwater recharge.

when water is applied to the surface of a soil, a part of it seeps into the soil. This movement of water through the soil surface is known as infiltration

INFILTRATION CAPACITY The maximum rate at which a given

soil at a given time can absorb water is defined as the infiltration capacity. It is designated as fc and is expressed in units of cm/h. The actual rate of infiltration f can be expressed as

f = fc when i > fc                                      f = i when i < fc

where i = intensity of rainfall. The infiltration capacity of a soil is high at the beginning of a storm and has an exponential decay as the time elapses. The infiltration process is affected by a large number of factors

MEASUREMENT OF INFILTRATION

Information about the infiltration characteristics of the soil at a given location can be obtained by conducting controlled experiments on small areas. The experimental set-up is called an infiltrometer.

There are two kinds of infiltrometers :• Flooding-type infiltrometer• Rainfall simulator.

• Flooding (ring) Infiltrometers

– Single ring– Double ring• Rainfall-Runoff Plot

Infiltrometers

Measuring Infiltration Rate

Flooding-Type lnfiltrometer

Water is poured into the top part to a depth of 5 cm and a pointer is set to mark the water level.

As infiltration proceeds, The volume is made up by adding water from a burette to keep the water level at the tip of the pointer.

Knowing the volume of water added at different time intervals, the plot of the infiltration capacity vs time is obtained. The experiments are continued till a uniform rate of infiltration is obtained and this may take 2-3 h.

This is a simple instrument consisting essentially of a metal cylinder, 30 cm diameter and 60 cm long, open at both ends. This cylinder is driven into the ground to a depth of 50 cm (Fig.3.10).

Modified Infiltrometer A major objection to the simple

infiltrometer as above is that the infiltered water spreads at the outlet from the tube (as shown by dotted lines in Fig. 3.10) and as such the tube area is not representative of the area in which infiltration takes place.

To overcome this a ring infiltrometer consisting of a set of two concentric rings (Fig.3.11) is used. In this two rings are inserted into the ground and water is maintained on the soil surface, in both the rings, to a common fixed level.

The outer ring provides a water jacket to the infiltering water of the inner ring and hence prevents the spreading out of the infiltering water of the inner tube. The measurements of water volume is done on the inner ring only.

Outer Rings are 6 to 24 inches in Diameter (ASTM - 12 to 24 inches)Mariotte Bottles Can be Used to Maintain Constant HeadRings Driven - 5 cm to 6 inches in the Soil and if necessary sealed

Disadvantages of InfiltrometersThe main disadvantages of flooding-type infiltrometer are :    • Raindrop-impact effect is not simulated;    • Driving of the tube or rings disturbs the soil structure;    • Results of the infiltrometer depend to some extent on their size with the larger meters giving less rates than the smaller ones; this is due to the border effect.

Rainfall Simulator In this a small plot of land, of about 2 m X 4 m size, is

provided with a size of nozzles on the longer side with arrangements to collect and measure the surface runoff rate.

The specially designed nozzles produce raindrops falling from a height of 2 m and are capable of producing various intensities of rainfall.

Experiments are conducted under controlled conditions with various combinations of intensities and durations and the surface runoff is measured in each case.

Using the water-budget equation involving the volume of rainfall, infiltration and runoff, the infiltration rate and its variation with time is calculated.

If the rainfall intensity is higher than the infiltration rate, infiltration-capacity values are obtained.

Rainfall simulator type infiltrometers given lower values than flooding type infiltrometers. This is due to the effect of the rainfall impact and turbidity of the surface water present in the former.

INFILTRATION-CAPACITY VALUES infiltration-capacity values of soils are

subjected to wide variations depending upon a large number of factors.

Typically, a bare, sandy area will have fc » 1.2 cm/h and a bare, clay soil will have fs » 0.15 cm/h.

A good grass cover or vegetation cover increases these values by as much as 10 times.

Classification of Infiltation capacities of soilsInfiltration Class

Infiltration Capacity (mm/hr)

Remarks

Very Low <2.5 Highly Clayey soilsLow 2.5 to 25 Shallow soils, Clay soils, soils

low in organic mattersMedium 12.5 to 25 Sandy loams, siltHigh >25 Deep sands, well drained

aggregated soils

Infiltration Percolation Rates by Soil Group/ Texture

Horton's equationHorton's equation is an empirical formula that says that infiltration starts at a constant rate, f0, and is decreasing exponentially with time, t. After some time when the soil saturation level reaches a certain value, the rate of infiltration will level off to the rate fc.

ft = fc + (f0 − fc)e − kt

Whereft is the infiltration rate at time t;f0 is the initial infiltration rate or maximum infiltration rate;fc is the constant or equilibrium infiltration rate after the soil has been saturated or minimum infiltration rate;k is the decay constant specific to the soil.

k= A constant(1/(m*log(2.73), Where m= slope of log (f-fc)versus time t(hr)

The other method of using Horton's equation is as below. It can be used to find the total volume of infiltration, F, after time t.

INFILTRATION- ExampleLocation Average infiltration details in Tata Plantsr.No Time

(minitues)Time (Hours)

cumulative.depth (cm)-Fp

Drop in water level (cm)

infiltration at each interval (cm/hr)-fp

1 0 0.00     2 30 0.500 5.800 5.80 11.603 60 1.000 10.2304 90 1.500 14.2005 120 2.000 17.7806 150 2.500 21.0507 180 3.000 24.0208 210 3.500 26.6409 240 4.000 29.040

0 0.5 1 1.5 2 2.5 3 3.5 402468

101214

infiltration (c...

Time (hrs)

Inflt

ratio

n (c

m/H

r)

0.00 0.20 0.40 0.60 0.80 1.000

0.51

1.52

2.53

3.5

f(x) = − 3.38968441405063 x + 3.22356965168785R² = 0.98900692524041

log(f-fc) vrs- Time

Time (hrs)Linear (Time (hrs))

log(f-fc)

Tim

e (h

rs)

f-fcLog (f-fc)

6.80 0.83

3.58 7.163.27 6.542.97 5.942.62 5.242.40 4.80

4.06 0.613.14 0.502.36 0.371.74 0.241.14 0.060.44 -0.360.00#NUM!

4.43 8.863.97 7.94

Results Infiltration Rate (cm/hr)

Cummulative infiltration (cm)

Infiltration for 4 hours (cm) 5.254 28.58Infiltration for 12 hours (cm) 4.802 67.65Infiltration for 24 hours (cm) 4.800 125.25

f= Infiltration capacity at any time tfc= The value of infiltration rate after it reaches a

constant valuefo= Infiltration capacity at the start Fp= Cummulative Infiltration at any time t(Fp)= k= A constant(1/(m*log(2.73), Where m= slope of

log (f-fc)versus time t(hr)t= time from beginning of the event

f= fc+(fo-fc)*e^(-k*t)Fp= ∫ fc+(fo-fc)*e^(-k*t) = fc*t+((f0-fc)/k)*(1-e^(-k*t)

0 0.5 1 1.5 2 2.5 3 3.5 402468

101214

infiltration Capacity of Soils infiltrati...

Time in hours

Infil

trat

ion

(cm

)

Data entry for estimating infiltration capacity of the soils ( Enter data only in Blue colour cells)

Location Averagesr.No Time

(minitues)Time (Hours)

cum_Drop in water level (cm)

drop in water level at each interval (cm_)

infiltration (cm/hr)

  0 0     1 30 0.5 5.80 5.80 11.6002 60 1 10.23 4.43 8.8603 90 1.5 14.20 3.97 7.9404 120 2 17.78 3.58 7.1605 150 2.5 21.05 3.27 6.5406 180 3 24.02 2.97 5.9407 210 3.5 26.64 2.62 5.2408 240 4 29.04 2.40 4.800

Results Infiltration Rate (cm/hr)

Cummulative infiltration

(cm)Infiltration for 4 hours (cm)  5.254 28.58Infiltration for 12 hours (cm) 4.802 67.65Infiltration for 24 hours (cm) 4.800 125.25

Infiltration Index(φ)Infiltration Index(φ)is defined as the rate of rainfall above which the rainfall volume equals the run off volume Runoff=Σ (I-φ)*∆ t

Runoff=Σ (I)* ∆ t - *∆ tΣ φ

Time from start (hrs)

0 2 4 6 8 10 12 14 16

Cumulative Rainfall (cm)

0 0.4 1.3 2.8 5.1 6.9 8.5 9.5 10

Given Runoff = 5.8 cm

φ (cm/hr) 0.2625

Runoff=Σ (I)* ∆ t - *∆ tΣ φ

5.8=10-16*φ

Σ I* ∆ t=10 Σφ*∆ t =16φRunoff=5.8 -=

Groundwater Separation. The required depth from the

bottom of the leach area to usable groundwater

About 4 to 10 % of infiltration rates are used for effluent dischargesEffluent rates should be less

than percolation rates

What Is Soil?• Soils are complex systems of solid

matter, pore spaces filled with water and oxygen, and numerous bacteria, fungi, and other organisms (Harris 1992).

• . Soil is a mixture of four basic components: – Inorganic materials (minerals),

including rock, clay, silt, and sand, give structure to the soil.

– Organic matter, including living and decomposing organisms and plant parts,

– Air that moves through the pore spaces provides oxygen to the roots.

– Water and dissolved nutrients, important for a number of the tree’s life processes, also move through the pore spaces.

Structure of soil

Grain size analysis

• A known weight of the sample was sieved through a number of different size sieves.

• The weight of the sample that did not pass through each sieve was recorded and converted to a percentage of the total sample weight.

Mesh Size

Size of sieve ( IN)

wt at different seives (gms)

Cummulative

weights retained(gms)

Cummulative % Retaine

d

% of sand,Silt,clay etc

14 0.046(Sand) 39 39 10 27.920 0.033(sand) 67 106 28 28 0.023(Silt) 46 152 40 51.135 0.016(silt) 75 227 60 48 0.012(Silt) 73 300 79 65 0.008( Clay) 42 342 90   Pan (bottom) 38 100 21.1 380 380 100.0

Sand analysis

Source: Brady, Nyle. 1990. The Nature and Properties of Soils

Soil Textural Classification

Soil Textural TriangleUSDA System

Soil water Plant relationshipDefinitions Field Capacity (FC)

• Field capacity is commonly defined as "The amount of water held in soil after excess water has drained away and the rate of downward movement has materially decreased". This usually takes place within 2 or 3 days after rain or irrigation in free draining soils

Permanent Wilting Point (PWP)• Not all the soil water stored in the root-zone can be extracted

by plants. Some of it is too strongly bound to the solid particles of the soil. This is called hygroscopic water. The soil water content at which 'plants have extracted all the water they can from a soil' is called the permanent wilting point. This varies depending on soil type and porosity.

Stress Point (SP) Long before the soil in the root

zone has dried out to the permanent wilting point, plant growth is severely retarded due to water stress. Plant leaves are subjected to a certain evaporative water demand by the prevailing climatic conditions (mainly the solar radiation and air temperature.)

If plants are to remain turgid, the roots must be able to extract water from the soil at a rate sufficient to match the evaporative demand.

A definition of the stress point is, 'the soil water content in the root zone at which plant growth starts to be significantly affected by water stress.'

Total Available Water-Holding Capacity (TAWC)

Once the plant is drawing water from below the stress point its growth is hampered. This is influenced by soil structure and texture.

This is defined as "the water held in the root-zone between field capacity and permanent wilting point". That is ALL the water which the plant can withdraw from the soil.

Readily Available Water-Holding Capacity (RAWC)

If plant growth rather than survival is of interest, or if irrigation is available, it is 'the water held in the root-zone between field capacity and the stress point' that is of interest. This may be called the readily available water-holding capacity and as a rough estimate is roughly half the TAWC.

• Method based on Agronomy– TAW= (FC-WP) * BD

*RZ * 10 MM– TAW=Total available

water (mm)– FC= Field Capacity(%)– WP= Wilting point(%)– BD= Bulk Density(%)– RZ= Root Zone depth

(m)

• Example• FC =21.85%• WP = 10%• BD = 1.35%• Rz = 0.9

Estimation of water Requirement

• TAW= (21.85-10) * 1.35 * 0.9 *10

• = 143.98 mm

crop water requirements . The basic formula for crop water

requirements is ETcrop = kc * Eto where:

• ETcrop = the water requirement of a given crop in mm per unit of time e.g. mm/day, mm/month or mm/season.)

• kc = the "crop factor" • ETo = the "reference crop

evapotranspiration" in mm per unit of time e.g. mm/day, mm/month or mm/season. (Eto= Kp* Epan)

ETcrop = kc *Kp* Epan

Available waterSoil Texture Storage   (mm water/100mm soil

depth)

Stones & gravel 0Sand 3Loamy sand 10Sandy loam 15Silt loam 20Clay loam 18Clay 16Peat 25

Soil Moisture Measuring Techniques Tensiometers• These sensors use a porous ceramic

cup attached to the bottom of a clear plastic tube/water reservoir and calibrated vacuum gage to measure soil moisture tension in centibars.

• Tensiometers come in varying lengths, from 1 foot to 4 feet in length,

• These devices are also soaked in water for at least one day before installation. Good contact between the ceramic cup and the surrounding soil is also essential for this device.

• As water flows out of the tensiometer into the surrounding soil until moisture equilibrates, it creates a partial vacuum in the tensiometer body which is read on the calibrated vacuum gage as matric potential or soil moisture tension.

Reading TensiometersThe tensiometer gauge reads the tension between soiland water particles. Soil moisture tension increaseswhen there is less water in the soil. As a result thetensiometer gauge, Figure 2, reads high for dry soilsand low for wet soils.A wet soil would be indicated by a reading under 10cbars and a reading above 50 cbars would indicate adry soil for most soil types.