1 Drought Planning for Vegetable Productioncemonterey.ucanr.edu/files/250808.pdf1 Drought Planning for Vegetable Production 2 Introduction 3 Under drought conditions when water may

Drought Planning for Vegetable Production 1

Introduction 2

Under drought conditions when water may be in short supply, growers often need to 3

estimate the potential water use of their crops to determine how much land can be irrigated. 4

Additionally, these calculations may be needed when switching to a different crop type under a 5

limited water supply. It is also necessary to estimate crop water requirements when buying or 6

leasing land to determine if a potential water supply is sufficient to irrigate the entire farm. The 7

following sections detail the calculations involved in estimating crop water requirements and 8

determining how much land can be irrigated under limited water supplies. Example calculations 9

are provided as well as a discussion of the key considerations in developing accurate estimates. 10

Defining crop water needs 11

The volume of irrigation water needed to produce a crop is the difference between crop 12

water needs and contributions of water from non-irrigation sources, such as precipitation from 13

rain and fog. Other non-irrigation sources of water include moisture contributed by shallow 14

water tables, and moisture stored in the soil profile. 15

Water is needed for crop production both pre-season (before planting) and in-season. 16

Water may be needed pre-season for preparing soil for planting and leaching salts, and during the 17

season for evapotranspiration, controlling salinity, and to compensate for inefficiencies in the 18

irrigation application. In some cases water is needed during the season for frost protection or for 19

increasing humidity and lowering the air temperature during hot periods. Each component of 20

in-season and pre-season crop water needs must be estimated to determine the overall water 21

needs of a vegetable crop. 22

Estimating in-season water needs 23

Crop evapotranspiration 24

Evapotranspiration (ET) is the water that crops lose by evaporation from the soil and wet 25

plant surfaces and through transpiration. Plants transpire water through stomata, small openings 26

on leaves, where liquid water transforms to vapor and is lost into the air. The daily ET rate is 27

usually expressed in inches or millimeters per day and depends on weather and crop factors. 28

Potential ET is the amount water a crop uses for evapotranspiration when it is fully 29

irrigated. Yields of vegetable crop are usually maximized when moisture is sufficient to meet the 30

potential ET rate throughout the season. A crop is under water stress when the supply of soil 31

moisture is insufficient to meet potential ET requirements. Short periods of water stress may 32

reduce the growth rate of crops. Prolonged periods of water stress will cause the pores on leaves 33

that transpire moisture, called stomata, to close to prevent water losses. The closure of stomata 34

reduces the efficiency of the crop to uptake and incorporate carbon dioxide into sugars. 35

Consequently, prolonged water stress will cause yield losses, especially if water stress occurs 36

during critical periods of crop development such as flowering or fruit development. ET of a crop 37

under water stress will be less than a crop watered to meet potential ET requirements. 38

Potential crop ET will vary depending on where and when a crop is grown, and the crop 39

type. Reference ET values are available for most agricultural regions of the state from the 40

California Irrigation Management and Information System (CIMIS) managed by the California 41

Department of Water Resources (CDWR) through their website (www.cimis.water.ca.gov). 42

Reference ET values are from weather stations located on a reference crop (Figure 1), usually 43

grass or alfalfa, with a consistent and full cover of vegetation. Historical daily reference ET 44

values are available for each region, as well as historical monthly averages of ET. The 45

reference ET values must be converted using crop coefficients to estimate ET for a different 46

crop. ANR publication 3396 explains how to estimate crop ET using crop coefficients. In 47

addition, a number of web-based programs can help calculate crop ET using reference ET values 48

and crop coefficients. WaterRight (www.wateright.org), Irrigation Scheduler Mobile 49

(weather.wsu.edu), and CropManage (cropmanage.ucanr.edu) are examples of online 50

applications that can be used to estimate ET of vegetable crops using CIMIS reference ET data. 51

Table 1 summarizes crop ET values for a range of vegetables produced on the Central Coast of 52

California. These values can be used as approximations for ET requirements for these 53

commodities for the Central Coast region. 54

Table 1. Estimated ET of selected vegetable crops grown on the Central Coast. 55

Region Crop

Irrigation

Method Season

ET

requirement

(inches)

Central Coast (coastal valleys)

Broccoli Sprinkler March - August 6

Broccoli Sprinkler May - September 7

Broccoli Sprinkler September - March 6

Brussels sprouts Sprinkler July - December 14

Cauliflower Sprinkler March - July 7

Cauliflower Sprinkler August - December 6

Cabbage Sprinkler August - October 10

Cabbage Sprinkler April - July 11

Celery Sprinkler/Drip May - September 8

Lettuce (iceberg) Sprinkler/Drip February - May 5

Lettuce (iceberg) Sprinkler/Drip April - July 7

Lettuce (iceberg) Sprinkler/Drip June - August 6

Central Coast (interior valleys)

Bok Choy (greenhouse) Sprinkler July-September 4

Broccoli Sprinkler March - August 13

Broccoli Sprinkler May - September 12

Broccoli Sprinkler September - March 6

Cabbage Sprinkler April - August 15

Cauliflower Sprinkler March -July 12

Cauliflower Sprinkler November - April 12

Celery Drip May - August 12

Lettuce (iceberg) Sprinkler/Drip April - July 10

Lettuce (iceberg) Sprinkler/Drip June - August 8

Lettuce (iceberg) Sprinkler June - August 9

Peppers Drip May - September 14

Spinach (Baby) Sprinkler May - September 3

Tomatoes (Fresh Market) Drip June -September 13

Tomatoes (Processing) Sprinkler/Drip May - September 14

56

Application efficiency 57

Application efficiency (also known as irrigation efficiency) needs to be factored into the 58

irrigation requirement of a crop. Application efficiency is defined as the fraction of applied 59

water that remains in the root zone where it can be used by the crop. A portion of applied water 60

may be lost to drainage or run-off. Both the distribution uniformity of an irrigation system and 61

scheduling of irrigations can affect application efficiency. Assuming that the irrigation schedule 62

is optimized by good management, distribution uniformity (DU) may be considered a close 63

approximation of application efficiency. A high DU means that an irrigation system applies 64

water uniformly in a field. Most well-designed and operated drip systems have a DU greater 65

than 85%. Well maintained and operated sprinkler systems have DU values ranging between 66

70% and 80%. Growers often allow for a low application efficiency during the establishment 67

of vegetable crops to assure that they attain a uniform stand of plants. Hence it may be 68

advisable to separate the application efficiency for establishment and post-establishment water. 69

Salt management 70

Leaching needed to control salinity in the root zone increases the water needs of a crop. 71

ANR publication 8550 entitled “Managing salts by leaching” describes how to estimate leaching 72

requirements (LR) of vegetable crops for specific water and soil salinity conditions. Leaching 73

can be accomplished both pre-season and during the cropping season depending on the water 74

availability and water quality. 75

The following equation can be used to estimate applied water needed to meet in-season 76

crop water requirements: 77

𝐴𝑊 = 𝐸𝑇𝑐 × 100%/𝐷𝑈

1 −𝐿𝑅

100%

78

where AW is applied water in inches, ETc is crop evapotranspiration in inches, DU is percent 79

distribution uniformity (%), and LR is percent leaching requirement. 80

81

Example: How much water will be needed for the in-season water requirements of summer 82

lettuce irrigated with sprinklers and grown in the interior valleys of the Central Coast if the 83

average DU is 70% and the leaching fraction needed is 10%? 84

According to Table 1, sprinkler-irrigated lettuce planted in June needs 9 inches of water 85

for ET. Using the above equation, 14.3 inches of applied water is needed to grow a lettuce crop. 86

𝐴𝑊 = 9 𝑖𝑛𝑐ℎ𝑒𝑠 × 100%/70%

1 −10%

100%

87

𝐴𝑊 = 14.3 𝑖𝑛𝑐ℎ𝑒𝑠 88

Crop establishment water 89

Vegetables often need more water than crop ET for germinating seeded crops or for 90

overcoming transplanting shock (Figure 2). The amount of establishment water will depend on 91

the existing moisture in the soil, length of time for germination or transplant establishment, 92

application efficiency, and weather conditions. In practice, most growers that plant or establish 93

vegetable transplants in moist soil will apply at least twice the reference ET × number of days for 94

germination or transplant establishment. 95

Example: What is minimum applied water (AW) needed for germinating a lettuce crop if 96

average reference ET = 0.17 inches per day and seed germination = 7 days? 97

AW for germination = 2 ×0.17 inches/day × 7 days = 2.4 inches 98

Frost protection, humidity, and temperature control 99

Irrigation is sometimes needed to influence the micro-climate of a crop for short periods. 100

Irrigation water applied with sprinklers can protect a crop from frost damage that may occur 101

during cold nights, increase humidity when hot dry winds blow, or decrease damage from 102

excessively hot conditions. It may be difficult to anticipate the amount of water needed for 103

affecting the micro-climate of a crop, but past experience and weather records may provide a 104

rough guide. 105

Estimating pre-season water needs 106

Water applied before planting needs to be factored into the crop water requirements. An 107

application of pre-season water may be needed to bring the soil to field capacity before planting 108

or tillage operations. If pre-season rainfall was insufficient, additional water may be needed to 109

leach salts that have built up in the soil. Often pre-season water can be minimized by evaluating 110

soil water status and determining the amount of water needed to bring the soil to saturation. 111

The required amount of pre-season water can be estimated by comparing the current water 112

content of the soil with the water content at field capacity. Table 2 estimates the water content of 113

soils of a range of textures at field capacity (~30 cbars). 114

Table 2. Texture effects on water holding capacity of soils. 115

116

Soil Texture

Field Capacity

(30 cbar)

Wilting Point

(1500 cbar)

Available

Moisture

----- inches of water per inch of soil depth-----

Sand 0.10 0.04 0.06

Loamy Sand 0.16 0.07 0.09

Sandy Loam 0.21 0.09 0.12

Loamy Sand 0.27 0.12 0.15

Silt Loam 0.30 0.15 0.15

Sandy Clay Loam 0.29 0.18 0.11

Sandy Clay 0.28 0.15 0.13

Clay Loam 0.32 0.18 0.13

Silty Clay Loam 0.36 0.20 0.16

Silty Clay 0.40 0.20 0.20

Clay 0.40 0.22 0.18

117

118

Example: If the water content of a clay loam soil averages 25% to a 3 foot depth, how much 119

water will be needed to bring the soil profile to field capacity? 120

Table 2 shows that the water content of a clay loam soil at field capacity is 32% or 0.32 inches 121

per inch of soil depth. The amount of water needed to bring the soil to field capacity equals: 122

(0.32 – 0.25) × 3 ft ×12 inches/ft = 2.5 inches 123

Estimating non-irrigation sources of water 124

Non-irrigation sources of water can satisfy a portion of the crop water needs. These 125

sources include water from rainfall, fog, and shallow water tables. Moisture stored in the soil 126

profile can also provide a portion of the water need to produce a vegetable crop. 127

Soil moisture contribution to crop water requirements 128

Even during a drought, a significant portion of the moisture contributing to crop ET may 129

be stored in the soil after pre-season rains or after pre-season irrigations. Soil moisture 130

contributing to crop water use is the difference in soil water content between planting and 131

harvest. 132

Soil moisture available for a crop can be estimated from the water-holding characteristics 133

of a soil. Soil water-holding characteristics describe the amount of water a soil can hold and how 134

tightly water is held at different water contents. Soils with a high percentage of clay and silt 135

sized particles (clay, clay loam) hold more water than soils with a high percentage of sand (sand, 136

sandy loam). Moisture in fine textured soils, such as clay loam soils, is held tighter than in 137

coarse textured soils such as sandy loam soils. A portion of the water held in the pores of a soil 138

is available to a crop and the remainder is unavailable because it is held too tightly for crop roots 139

to extract. Generally, 15 bars is used as a standard tension at which most plants cannot extract 140

water from soil pores. Available water is the difference between the water content of the soil at 141

field capacity (water content of a soil after it is saturated and excess water drains) and the water 142

content at wilting point. Table 2 provides estimates of available water for soils of a range of 143

textures. Allowable depletion is the percentage of available soil water that can be taken up by a 144

crop before the soil water content is so low that production and yield are adversely affected. 145

Table 3 summarizes allowable depletions for common vegetable crops. For the purpose of 146

estimating the change in soil moisture storage over a crop cycle, the allowable depletion should 147

equal the soil moisture content at harvest. Rooting depth at harvest (Figure 3) also needs to be 148

considered in the estimate because the greater the root depth the more soil moisture that the crop 149

can extract. Table 4 summarizes common rooting depths of vegetable crops. 150

151

Table 3. Allowable depletion of available soil moisture at crop maturity. 152

Crop

Allowable depletion

of available soil

moisture (%)1 Crop

Allowable depletion

of available soil

moisture (%)1

Artichoke 45 Lettuce (leaf) 25

Asparagus 40 Lettuce (baby) 25

Bean (green) 30 Melons (mixed) 60

Bean (snap) 30 Okra 65

Beet (table) 50 Onion (dry) 40

Broccoli 40 Onion (green) 25

Brussels sprout 50 Parsnip 40

Cabbage 35 Pea 30

Cantaloupe 40 Pepper (Bell) 25

Carrot 35 Potato 35

Cauliflower 40 Potato (sweet) 65

Celery 25 Pumpkin 50

Chard 30 Radish 35

Cilantro (Coriander) 30 Spinach 35

Corn (sweet) 30 Spinach (baby and teen) 25

Cucumber 50 Squash (summer) 35

Eggplant 30 Squash (winter) 60

Garlic 50 Tomato (fresh) 35

Kale 50 Tomato (processing) 60

Leek 35 Turnip (white) 35

Lettuce (iceberg) 30 Watermelon 40

Lettuce (romaine) 30

1. Sources: Chapt. 15. Vegetable Irrigation, Irrigation of Agricultural Crops, ASA Monograph # 153

30., 2nd Ed. Hanson et. al. 2004. Scheduling Irrigations. ANR pub 3396. M. Cahn (personal 154

communication with growers, consultants, and farm advisors) 155

156

Table 4. Range of rooting depths of common vegetables at maturity. 157

Crop

Rooting Depth

at Maturity1 (ft) Crop

Rooting Depth

at Maturity1 (ft)

Artichoke 2.0 - 3.0 Lettuce (leaf) 1.0 - 1.5

Asparagus 2.5 - 3.5 Lettuce (baby) 1.0 - 1.5

Bean (green) 1.5 - 2.0 Melons (mixed) 2.0 - 4.0

Bean (snap) 1.5 - 2.0 Okra 2.0 - 3.0

Beet (table) 1.0 - 1.5 Onion (dry) 1.5 - 2.0

Broccoli 2.0 - 3.0 Onion (green) 1.0 - 1.5

Brussels sprout 2.0 - 3.0 Parsnip 2.0 - 3.0

Cabbage 2.0 - 3.0 Pea 1.5 - 2.0

Cantaloupe 2.0 - 4.0 Pepper (Bell) 2.0 - 3.0

Carrot 1.5 - 2.0 Potato 1.5 - 2.0

Cauliflower 2.0 - 3.0 Potato (sweet) 2.0 - 3.0

Celery 1.5 - 2.0 Pumpkin 3.0 - 4.0

Chard 2.0 - 3.0 Radish 1.0 - 1.5

Cilantro (Coriander) 1.5 - 2.0 Spinach 1.5 - 2.0

Corn (sweet) 3.0 - 4.0 Spinach (baby and teen) 1.0 - 1.5

Cucumber 1.5 - 2.0 Squash (summer) 2.0 - 3.0

Eggplant 2.0 - 3.0 Squash (winter) 2.5 - 3.5

Garlic 1.5 - 2.0 Tomato (fresh) 2.0 - 4.0

Kale 2.0 - 3.0 Tomato (processing) 3.0 - 5.0

Leek 1.5 - 2.0 Turnip (white) 1.5 - 2.5

Lettuce (iceberg) 1.5 - 2.0 Watermelon 2.0 - 3.0

Lettuce (romaine) 1.5 - 2.0

1. Source: Chapt 11. Sprinkler Irrigation Section 15, Soil Conservation Service National 158 Engineering Handbook, Chapt. 15. Vegetable Irrigation, Irrigation of Agricultural Crops, ASA 159 Monograph # 30. 2nd Ed. M. Cahn (personal communication with growers, consultants, and farm 160 advisors) 161

162

Example: How much soil moisture is available to a lettuce crop planted on clay loam soil? 163

Available moisture for a clay loam soil is approximately 0.13 inches per inch depth of 164

soil. The allowable depletion of available soil moisture for lettuce is 30%, and roots of a typical 165

lettuce crop reach a depth of 24 inches (2 feet) by maturity. 166

Available soil moisture = 0.13 inches per inch of soil × 30%/100% × 24 inches 167

= 0.9 inches 168

Precipitation and shallow water tables 169

Other sources of water that can contribute to crop ET include rainfall, fog, and shallow 170

water tables. Rainfall can be significant during the winter, early spring or late fall in some 171

regions of the state but is difficult to predict. Long term rainfall records, available from CIMIS 172

and other agencies, can be used to give a general sense of how much rain is typical, but using a 173

conserve estimate is recommended. Near the coast, fog often contributes precipitation to crops 174

during the summer. Amounts are usually less than a few tenths of an inch per year in most 175

regions of the coast. Water from perched or shallow water tables can move upward into the root 176

zone of a crop. The amount of water contributed from a shallow water table is difficult to 177

assess, but may depend on the soil type, rooting pattern of the crop, and when the water table is 178

present during the crop cycle. If the water table is more than 2 feet below the rooting zone, the 179

contribution of moisture is probably negligible. 180

Determining the overall irrigation water requirement 181

The overall irrigation requirement for a crop based on the above examples can be 182

estimated by summing the pre-season and in-season water needs and subtracting for non-183

irrigation water sources: 184

185

Crop water needs

Pre-season water 1.5

In-season water

Water for establishment 2.4

ETc, Leaching, DU 14.3

Temperature and humidity 0.0

Total 18.2

Non-irrigation water sources

Change in available soil moisture 0.9

Rainfall 1.0

Fog 0.1

Shallow ground water 0.0

Total 2.0

Irrigation water requirement

Crop water needs 18.2

Non-irrigation water sources 2.0

Irrigation water needed 16.2

186

Determine how much water will be available for irrigation 187

In addition to estimating the crop water needs, one needs an estimate of the amount of 188

water available for irrigating during the upcoming season for calculating the acres that can be 189

farmed. Water availability will depend on the source of the water, water rights, and local and 190

state regulations. Water districts that are allocated surface water may not know the final water 191

allotment until late in the spring when most of the precipitation for the season has occurred. In 192

this case, it may be most useful to estimate the land that can be planted for a range of water 193

allocation scenarios (20%, 30%, 40%, 50%, etc. allocations). 194

Growers that rely on ground water for irrigation should conduct a pump test to determine 195

how much water their wells can produce. If aquifer levels have declined during a drought, then 196

wells will likely produce less water. A pump test will determine the flow rate (gallons per 197

minute) that a well can produce at different output pressures. If high pressure sprinklers will be 198

used for irrigating, a pump will have a lower output flow rate than if drip tape, which requires 199

less pressure, will be used. 200

Calculating the total acres that can irrigated 201

The maximum acres that can be irrigated is determined by dividing the total available 202

water by the water requirement of the crop. For example, if a grower is allocated 500 acre-ft of 203

water, then for the above example he can farm 370 acres of lettuce: 204

𝐶𝑟𝑜𝑝𝑝𝑒𝑑 𝑎𝑟𝑒𝑎 = 500 𝑎𝑐𝑟𝑒 ∙ 𝑓𝑡 × 12 𝑖𝑛𝑐ℎ𝑒𝑠/𝑓𝑡

16.2 𝑖𝑛𝑐ℎ𝑒𝑠 205

𝐶𝑟𝑜𝑝𝑝𝑒𝑑 𝑎𝑟𝑒𝑎 = 370 𝑎𝑐𝑟𝑒𝑠 206

Allowances for Peak ET 207

In addition to calculating the area that can be irrigated using the total amount of water 208

available for the season, one should also calculate the area that can be irrigated based on the 209

amount of water available per day during the period when the crop has the greatest 210

evapotranspiration requirement. Historical CIMIS ET data can provide maximum values for 211

reference ET during the growing season. The maximum crop coefficient value, which should 212

range between 0.8 and 1.15 depending on the crop, should be used to calculate the peak ET 213

requirement. Bulletin?? 214

Example: How many acres could be cropped if a ranch has 2 wells with a combine 215

output of 1500 gpm, and the peak crop ET is expected to be 0.28 inches/day in the summer? 216

Assuming that the wells can be operated 24 hours per day: 217

𝐶𝑟𝑜𝑝𝑝𝑒𝑑 𝑎𝑟𝑒𝑎 = 1500 𝑔𝑝𝑚 × 24 ℎ𝑜𝑢𝑟𝑠

0.28 𝑖𝑛𝑐ℎ𝑒𝑠 × 453 218

𝐶𝑟𝑜𝑝𝑝𝑒𝑑 𝑎𝑟𝑒𝑎 = 284 𝑎𝑐𝑟𝑒𝑠 219

where 453 in the above equation is a combined constant for converting gallons to acre inches and 220

hours to minutes. 221

Summary 222

Under drought or when water supplies are limited, estimating the water requirement of 223

crops is necessary to avoid planting more land than can be irrigated. Both pre-season and in-224

season water requirements should be estimated. The major crop water needs in-season are for 225

stand establishment, evapotranspiration, and managing salinity through leaching. The pre-season 226

water needs are mainly for leaching salts that may have built up in the soil from previous crops, 227

and for providing optimal moisture for tillage operations. Non-irrigation sources of water such 228

as precipitation and changes in stored soil moisture can be subtracted from the overall irrigation 229

requirements of the crop. Dividing the total amount of water available for irrigation by the crop 230

water requirement provides an estimate of the total acres that can be productively farmed. 231

Further Reading 232

Cahn, M., and K. Bali. 2016. Drought tip: “Managing salts by leaching” ANR #8550 pp. 8. 233

Hanson, B., L. Schwankl and A. Fulton 2004. Scheduling irrigations: when and how much 234

water to apply. ANR publication 3396. pp. 202. 235

Lascano, R.J. and R.E. Sojka, 2007. Irrigation of agricultural crops. American Society of 236

Agriculture. Madison, Wisconsin, USA. pp. 664. 237

Hanson, B. 2000. Irrigation pumping plants. ANR publication 3377. pp. 126. 238

239 Figure 1. Weather station used to estimate reference evapotranspiration. Photo credit: M. Cahn. 240

241

242

243

244

245

246

Figure 2. Overhead sprinklers are used to irrigate recently transplanted vegetables. Photo credit: 247 M. Cahn. 248

249

Figure 3. A mature iceberg lettuce crop may have roots below 2 feet of depth. Photo credit: M. 250

Cahn. 251

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255