Review Article t

Drip provides the salinity control needed for profitable irrigation of tomatoes in the San Joaquin Valley

by Blaine R. Hanson, Don E. May, Jirka Šimunek,̊ Jan W. Hopmans and Robert B. Hutmacher

Despite nearly 30 years of research supporting the need for subsurface -water disposal facilities, the Subsurface drainage systems and drainage-water lack of these facilities continues to disposal methods are not needed for properly plague on the San Joaquin designed and managed systems. Valley’s west side. One option for coping with the resulting salinity and shallow water-table problems is to convert from furrow or sprinkle irrigation to drip irrigation. Com- mercial field studies showed that subsurface drip systems can be highly profitable for growing processing tomatoes in the San Joaquin Valley, provided that the fraction can achieve adequate salinity control in the root zone. Computer simula- tions of water and salt movement showed localized leaching fractions of about 25% under subsurface drip irrigation, when water applications equaled the potential evapo- transpiration. This research suggests Subsurface drip irrigation is allowing San Joaquin Valley tomato growers to apply water precisely and uniformly, increasing yields and reducing the runoff of saline drainage water. that subsurface drip irrigation can be successfully used in commercial fields A UC study (Schoups et al. 2005) as a result, subsurface drip irrigation without increasing root-zone soil concluded that a salt balance must be is commonly used in salt-affected salinity, potentially eliminating the maintained in the root zone for produc- for processing tomato production. The need for subsurface drainage-water tive cropping systems to continue, and second option has been proposed, but irrigation without improved manage- little information exists on its use by disposal facilities. ment practices cannot be sustained growers. The California Department of in the San Joaquin Valley. The only Water Resources is promoting the third he lack of widespread subsurface options available to address salin- option, but its use is limited and still in drainage-water disposal facilities ity and drainage problems without an experimental stage. Tcontinues to plague agriculture along retiring land are: (1) reducing drain- One way to implement option one is the west side of the San Joaquin Val- age through the better management to convert from furrow or sprinkle ir- ley. Despite more than 30 years of of irrigation water; (2) increasing the rigation to drip irrigation. Drip irrigation research, drainage-water disposal use of shallow for crop applies water precisely and uniformly at methods that are economically, techni- irrigation without any yield reduc- high frequencies, potentially increasing cally, politically and environmentally tions; and (3) reusing drainage water. yield and reducing root-zone soil salin- feasible have not been implemented. In All three methods require adequate ity and drainage. These advantages are some areas, land retirement has been salinity control in the root zone. This not only governed by the technology, but the result. study is an example of the first option; also by the design, installation, opera-

http://californiaagriculture.ucanr.org • July–September 2009 131 Subsurface drainage systems and drainage-water Between 1998 and 2003, experi- disposal methods are not needed for properly ments in commercial fields in the designed and managed drip irrigation systems. Westlands Water District, on the San Joaquin Valley’s west side, evaluated subsurface drip irrigation of process- tion and maintenance of drip systems. rigation of assume an additional ing tomatoes under saline, shallow The main disadvantage of drip irrigation economic risk. groundwater conditions. In addition, is its high installation cost, which ranges In 2008, the Westlands Water starting in 2006, computer simula- from $600 to $1,000 per acre. Subsurface District — which encompasses more tions using the HYDRUS-2D model drip irrigation, commonly used for pro- than 600,000 acres of farmland in (Šimůnek et al. 1999) evaluated leaching cessing tomatoes, involves placing drip western Fresno and Kings counties — with subsurface drip irrigation under lines 8 to 12 inches below the soil surface reported 37,396 acres of cotton and these conditions. This model has been directly below the plant row; surface 86,011 acres of processing tomatoes, used previously in studies of water drip irrigation involves placing the drip now the largest single crop acreage; and chemical movement under drip ir- lines on the soil surface. cotton production has decreased sub- rigation (Gärdenäs et al. 2005; Hanson, In the late 1980s, two large-scale stantially in recent years (Westlands Šimůnek, et al. 2006). We present a re- comparisons of subsurface drip and Water District 2009). Because process- view of this research. furrow irrigation were conducted in ing tomatoes are a higher value crop Commercial field experiments cotton under saline, shallow groundwa- than cotton, subsurface drip irriga- ter conditions (Fulton et al. 1991; Styles tion offers potentially higher profits. Experiments in three commercial et al. 1997). Drip irrigation consistently However, unlike cotton, tomatoes are fields (sites BR, DI and DE) compared resulted in higher cotton yields with moderately sensitive to , subsurface drip irrigation to sprinkle less water application than furrow ir- and reduced tomato yields can result. irrigation (Hanson and May 2003, 2004). rigation. However, the profit with fur- The threshold electrical conductivity Drip systems ranged from 40 to 80 row irrigation was much higher at one (EC), which represents the maximum acres each in area, and sprinkle irriga- location, and drip irrigation was only root-zone soil salinity at which yield tion was used for the rest of the fields. slightly more profitable at the other. is not reduced, is 2.5 deciSiemens per depths ranged from 2 to The cost of the drip systems played the meter (dS/m) for tomato compared to 6 feet. Electrical conductivity ranged major role in their profitability. As a 7.7 dS/m for cotton (Mass and Grattan from 0.3 dS/m for irrigation water from result, growers who convert to drip ir- 1999). Westlands Water District to 1.1 dS/m for water, and from 4.0 to 16.4 dS/m in the shallow groundwater. A small-scale, randomized, replicated experiment was conducted in each drip-irrigated field to investigate the relationship between

Andros Engineering yield, soluble solids (a measure of yield quality) and applied water. The soil type was loam at the three experi- mental sites. We found that subsurface drip ir- rigation was highly profitable for pro- cessing tomatoes under these shallow, saline groundwater conditions com- pared to sprinkle irrigation. Average yields were 40.5 tons per acre for sub- surface drip irrigation versus 33.9 tons per acre for sprinkle irrigation, with $484 per acre more profit on average for drip than sprinkle irrigation. The average difference in soluble solids between the two irrigation methods was not significant. The small-scale experiments showed increased yield and decreased soluble solids as applied Specialized equipment (shown here, by Andros Engineering) is used to install drip tape 8 to 12 inches below the soil surface, at a cost of about $600 to $1,000 per acre. Despite this water increased. price, studies show that improved irrigation efficiency and yield benefits increase profits for Yields of the drip-irrigated fields growers in the San Joaquin Valley, compared with sprinkle or furrow irrigation. were monitored for 2 more years after

132 CALIFORNIA AGRICULTURE • VOLUME 63, NUMBER 3 the first year. Yields remained high water was 8 to 10 dS/m (the threshold Electrical conductivity (dS/m) except for one site, which had 2 years for cotton is 7.7 dS/m). of reduced yields due to late plantings. At all commercial sites, tomato yields 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

We did not find any trends toward yield increased as applied water increased. Plant row reductions with increased soil salinity Factors contributing to this finding A near the drip lines, which ranged from included higher soil- and –5 values less than, to higher than, the reduced root-zone soil salinity due to –10 Drip line threshold electrical conductivity of 2.5 larger zones of low salt around the drip –15 dS/m for tomatoes. lines as more water was applied. Cotton –20 At a fourth commercial field (site yields, however, were unresponsive to BR2), a small-scale, randomized-block, the amount of applied water, reflect- –25 replicated experiment evaluated the ing cotton’s salt tolerance and ability B response of tomato and cotton yields to utilize saline, shallow groundwater –5 to different amounts of applied wa- (Wallender et al. 1979). Consequently, –10 ter under very shallow groundwater contributions by the saline, shallow –15 conditions of 18 to 24 inches (Hanson, groundwater to crop evapotranspira- –20 Depth (inches) Hutmacher, et al. 2006). The soil type tion should be minimized for tomato –25 was clay loam. Tomato yields ranged and maximized for cotton. from 34.6 tons per acre for 15.6 inches Soil salinity levels around the drip –5 C of applied water to 42.8 tons per acre lines depended on the depth to ground- –10 for 23.2 inches, even though near- water, salinity of shallow groundwater, saturated, highly saline soil occurred salinity of irrigation water and amount –15 at only 18 inches deep. At 23.2 inches, of applied water. For a water table –20 water application is about equal to the depth of about 6 feet, relatively uni- –25 seasonal or crop form soil salinity occurred throughout –25 –20 –15 –10 –5 0 5 10 15 20 25 water use for tomatoes. However, cot- the profile, with values smaller than Distance from drip line (inches) ton yields did not respond when water the threshold electrical conductivity of was applied at amounts equal to or tomato (fig. 1A). For water table depths Fig. 1. Soil salinity/electrical conductivity (EC) greater than about 40% of the poten- less than about 3 feet, relatively low around the drip line for water depth of about (A) 6 feet, EC irrigation water = 0.3 dS/m, EC tial seasonal evapotranspiration. The levels of soil salinity occurred near groundwater = 8 to 11 dS/m; (B) 2 to 3 feet, EC electrical conductivity of the irrigation the drip line, but values increased to irrigation water = 0.3 dS/m, EC groundwater = 5 water was 0.5 dS/m and of the ground- high levels beyond the wetting pat- to 7 dS/m; and (C) 2 to 3 feet, EC irrigation water = 1.1 dS/m, EC groundwater = 9 to 16 dS/m. tern due to the upward flow of shallow groundwater (figs. 1B and 1C). Higher TABLE 1. Seasonal applied water, evapotranspiration and leaching fractions soil salinity occurred near the drip line calculated from a for four when the salinity of the irrigation wa- Electrical conductivity (dS/m) commercial sites ter increased (fig. 1C). Larger amounts of applied water increased the zone of 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Seasonal Seasonal Leaching low-salt soil near the drip line, even Plant row Year* applied water ET† fraction‡ when shallow water tables had depths ...... inches ...... % A (23.2 inches) Drip line BR of less than 2 feet (fig. 2). –10 1999 16.0 20.3 0 At all sites, water table depth 2000 16.8 21.4 0 showed little response to drip irriga- –20 2001 20.5 22.9 0 tion, except when overirrigation oc- DI curred during one year at site BR (data 1999 22.2 25.1 0 B (15.6 inches) 2000 29.0 25.2 13.1 not shown). A subsequent reduction

2001 22.9 26.6 0 in applied water at that site caused the Depth (inches) –10 DE water table to decline due to reduced 2000 28.8 24.2 13.6 percolation and the natural drainage of –20 2001 22.1 23.1 0 shallow groundwater. BR2 –30 –25 –20 –15 –10 –5 0 5 10 15 20 25 30 2002 23.2 24.3 0 Determining leaching fractions Distance from drip line (inches) * BR, DI, DE and BR2 are site designations for the commercial fields. Salinity control is needed in the Fig. 2. Soil salinity/electrical conductivity (EC) † Evapotranspiration. root zone to maintain profitable sub- around the drip line for water depth of about ‡ Zero values indicate no leaching, which occurred because surface drip irrigation of tomatoes in seasonal applied water values were smaller than seasonal 18 to 24 inches, EC irrigation water = 0.5 dS/m, evapotranspiration. salt-affected soils. This can be achieved EC groundwater = 8 to 10 dS/m, for water by leaching or flushing salts from the applications of (A) 23.2 and (B) 15.6 inches.

http://californiaagriculture.ucanr.org • July–September 2009 133 Electrical conductivity (dS/m) a daily evapotranspiration rate of 0.29 0 1 2 3 4 5 6 7 8 9 10 11 12 inches per day, but the actual simula- 0 tions varied by applied water amounts –5 Drip and irrigation frequency. The applica- –10 line tion rate was constant during the simu- –15 lation period for a particular scenario –20 consisting of a water table depth, an –25 irrigation water salinity and an applied Depth (inches) –30 water amount. –35 A B C D We simulated two per 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 week for a 40-inch water table depth, Distance from drip line (inches) and daily irrigations for the 20-inch depth. These frequencies reflect those Fig. 3. Soil-water salinity/electrical conductivity (EC) around the drip lines at (A) start of used in the commercial field experi- simulation period (T = 0 day), (B) just after first irrigation (T = 1 day), (C) just before second irrigation (T = 3.5 days) and (D) just after last irrigation (T = 39.5 days). Applied water = 100% ments (Hanson et al. 2003). The drip evapotranspiration; EC irrigation water = 0.3 dS/m. line was 8 inches deep, and electrical conductivity of the shallow ground- root zone — applying irrigation water long drip irrigation can be sustained water was 10.0 and 8.0 dS/m for the in amounts exceeding the under saline, shallow groundwater 20- and 40-inch water table depths, depletion. The leaching fraction is used conditions. The soil salinity data, how- respectively, based on measured levels to quantify leaching adequacy, and is ever, clearly showed that because of the in the commercial fields. The initial derived from the ratio of the amount of wetting pattern under drip irrigation, soil-water salinity levels at the start water that drains below the root zone to leaching was highly concentrated near of the simulation period were based the amount of water applied. the drip line (referred to as “localized on samples collected in spring, prior Leaching fractions can be determined leaching”). The soil salinity data also in- to drip irrigation. The simulated root several ways. One approach is to mea- dicated that the water balance approach distribution was based on field data sure the average salinity of the root-zone is not appropriate for drip irrigation of rooting patterns for drip-irrigated soil and irrigation water, and then use and that estimating actual or localized tomatoes at the UC West Side Research appropriate charts or equations to deter- leaching fractions under drip irrigation and Extension Center (Hanson and mine the leaching fraction. However, soil may be difficult and also inaccurate. It May 2007). salinity, soil-water content and root den- is reasonable to expect that the salin- Simulated reclamation (salt removal) sity all vary around the drip line, result- ity patterns reflect long-term behavior, of soil near the drip line was rapid, ing in uncertainty about the accuracy of as long as adequate salinity-control and the simulated salinity patterns root-zone soil salinity. measures (sufficient leaching and no were consistent with those found in A second approach commonly groundwater intrusion into the root the commercial fields (fig. 3) (Hanson used is the water balance method, by zone) prevent salts from accumulating et al. 2008). The simulations predicted which a fieldwide amount of leaching in the root zone. that the volume of reclaimed soil would is calculated as the difference between increase over time, with most reclama- Computer simulations the seasonal amount of applied water tion occurring below the drip line, and (measured with a flow meter) and evapo- Because of the difficulties in estimat- that salts would accumulate near the transpiration. Because actual evapo- ing actual leaching fractions for the soil surface. Large seasonal applications transpiration in a given field is usually drip-irrigated commercial fields, we of water would increase the zone of unknown, it is frequently estimated us- used the computer model HYDRUS-2D lower-salinity soil near the drip lines, ing crop coefficients and a reference crop (Šimůnek et al. 1999) to simulate the consistent with our field data. But the evapotranspiration value obtained from movement of water and salt in soil un- larger amounts would have little effect the California der drip irrigation for a 42-day period on the volume of reclaimed soil above Information System (CIMIS). and quantify drainage below the root the drip line. As expected, salinity near We calculated fieldwide leaching zone. Simulations were conducted for the drip line would increase as irriga- fractions for the commercial fields water table depths of 20 and 40 inches; tion water salinity increased. The root using the water balance method. irrigation water salinities of 0.3, 1.0 and uptake of soil water would decrease as Evapotranspiration was determined 2.0 dS/m; and applied water at 80%, applied water decreased, suggesting using canopy growth rates and a cali- 100% and 115% of potential evapotrans- the potential for decreased yields with brated computer model. These calcu- piration. For 0.3 dS/m irrigation water, decreasing water applications, as was lations showed little or no fieldwide we conducted an additional simulation found in our commercial field data for leaching at most of the sites (table 1), of applied water at 60% of potential processing tomatoes. which suggests inadequate salinity evapotranspiration. The depth of ap- The actual or localized leaching control and raises questions about how plication per irrigation was based on fractions for the 40-inch water table

134 CALIFORNIA AGRICULTURE • VOLUME 63, NUMBER 3 scenarios were 7.7% for the 60% water In both studies (field experiments increased as the amount of applied wa- application treatment, 17.3% for the 80% and computer simulations), consider- ter increased, and soil salinity around treatment, 24.5% for the 100% treatment able localized leaching occurred around the drip line increased as salinity of the and 30.5% for the 115% treatment. As the drip lines, due to the wetting pat- irrigation water increased. irrigation water salinity increased, the terns of subsurface drip irrigation. The We found that very high irrigation actual leaching fraction increased as localized or actual leaching fractions efficiencies under drip irrigation can a result of reduced root-water uptake. determined from the computer simula- only be obtained by substantial deficit Even for water applications equal to or tions were about 25% to 30% for a water irrigation, in contrast to the frequent as- smaller than 100% of potential evapo- application equal to 100% of potential sumption that drip irrigation is nearly transpiration, drainage occurred below evapotranspiration. 100% efficient for water applications the root zone due to the spatially vari- Under subsurface drip irrigation of equal to about 100% of potential evapo- able wetting under drip irrigation. processing tomatoes, localized leach- transpiration. A common assumption is that ap- ing is highly concentrated near the Sustainable drip irrigation plying water at amounts equal to 100% drip line, resulting in relatively low of potential evapotranspiration results soil-salinity levels in areas where root The key to sustained subsurface in irrigation efficiency of 100%, defined density is highest. The water balance ap- drip irrigation of processing tomatoes as the ratio of cumulative root-water proach for estimating leaching amounts in salt-affected soils is profitability, uptake to applied water. In cases of is inappropriate for drip irrigation be- which in turn depends on salinity con- drip irrigation at 100% of potential cause of such localized leaching. trol in the root zone. This requires ir- evapotranspiration, little drainage be- The computer simulations showed rigating with relatively low-salt water; low the root zone is assumed to occur. that reclamation around drip lines in applying sufficient irrigation water for However, the computer simulations saline soil would be rapid. Predicted adequate localized leaching; leaching showed that this assumption is not reclamation was faster for relatively salts that accumulate around the drip true. Because of spatially varying soil- infrequent large water applications per line; and preventing saline, shallow water wetting around the drip lines, ir- irrigation than for smaller applications. groundwater intrusion into the root rigation efficiency was 74.6% and 69.7% The low-salt zone around the drip line zone. The following are recommenda- for the 40- and 20-inch water table scenarios, respectively, with the 100% water application. Very high irrigation efficiencies occurred only under condi- tions of severe deficit irrigation. Because of high-frequency irriga- tion, the volume of drainage per ir- rigation was small and drainage was distributed evenly over the irrigation season. As a result, natural subsurface drainage in the commercial fields was sufficient to prevent groundwater in- trusion into the root zone. Leaching and efficient drip systems The field research and computer simulation modeling demonstrated that subsurface drip irrigation of pro- cessing tomatoes is highly profitable compared to sprinkle or furrow irriga- tion under saline, shallow groundwa- ter conditions. Tomato yields increased as applied water increased, and cotton yields were unaffected. These tomato yield results suggest that root uptake of saline, shallow groundwater should be minimized to prevent yield reduc- tions, while the cotton yield results indicate that substantial root uptake To minimize the uptake of shallow, saline groundwater — which can affect tomato yields — of the saline groundwater can occur sufficient irrigation water must be applied in the root zone to ensure adequate leaching. without yield reductions. Above, filters, pumps and fertilizer tanks are part of drip irrigation systems.

http://californiaagriculture.ucanr.org • July–September 2009 135 tions for subsurface drip irrigation of plications of water per irrigation and little research has been conducted in the processing tomatoes under conditions relatively uniform distribution of irriga- San Joaquin Valley on drip irrigation of of the San Joaquin Valley’s west side: tions over time, coupled with natural salt-sensitive under saline condi- Water applications. Seasonal water subsurface drainage, prevented ground- tions, a literature review of numerous applications should be about equal to water intrusion into the root zone. This studies on drip irrigation of the seasonal evapotranspiration, which finding suggests that, for the conditions crops (Hanson et al. 2008) showed that is 25.5 inches in the San Joaquin Valley in these fields, subsurface drainage drip irrigation may be a sustainable (Hanson and May 2006). This provides systems and drainage-water disposal practice for salt-sensitive crops. sufficient localized leaching. Higher methods are not needed for properly applications could raise the water table, designed and managed drip irrigation causing saline, shallow groundwater systems. B.R. Hanson is Irrigation and Drainage Specialist, intrusion into the root zone. Smaller ap- These results indicate that sub- Department of Land, Air and Water Resources, plications reduce tomato yields. surface drip irrigation of processing UC Davis; D.E. May is Farm Advisor Emeritus, UC Salinity of irrigation water. The elec- tomatoes — a higher value, moderately Cooperative Extension; J. Šimunek̊ is Professor of trical conductivity of irrigation water salt-sensitive crop compared to cot- Soil Physics and Hydrologist, Department of Envi- should be about 1.0 dS/m or less; higher ton — is sustainable in the salt-affected ronmental Sciences, UC Riverside; J.W. Hopmans levels may reduce yields. soils that we studied. Similar results is Professor of Water Management, Department Irrigation frequency. From daily might be expected for crops of similar of Land, Air and Water Resources, UC Davis; and R.B. Hutmacher is Extension Cotton Specialist, De- to two or three irrigations per week value that are moderately salt sensitive partment of Plant Sciences, UC Davis, and Direc- should occur after the start of drip ir- and suitable for drip irrigation, such as tor, UC West Side Research and Extension Center. rigation (Hanson et al. 2003). Daily melon. Drip irrigation of salt-tolerant Support for this project was provided by the UC irrigations are recommended for very crops such as cotton, sugar beets and Prosser Trust Fund; the Westlands Water District; shallow, saline groundwater conditions. grain may not be profitable because of and Farming D and Britz Farming Company, both The amount of water application per their relatively low cash value. While of Five Points, Calif. irrigation should be determined using appropriate crop coefficients (Hanson and May 2006) and the reference crop evapotranspiration from CIMIS. References fertigation under microirrigation. Ag Water Mgt Salt leaching. Periodic leaching of Fulton AE, Oster JD, Hanson BR, et al. 1991. 86:102–13. salt accumulated above buried drip Reducing drain water: Furrow vs. subsurface drip irrigation. Cal Ag 45(2):4–8. Hanson BR, Šimunek̊ J, Hopmans JW. 2008. Leach- lines will be necessary with sprinkle ir- ing with subsurface drip irrigation under saline, rigation for stand establishment, if win- Gärdenäs A, Hopmans JW, Hanson BR, Šimunek̊ J. shallow groundwater conditions. Vad Zone J 2005. Two-dimensional modeling of nitrate leach- 7(2):810–8. ter and spring rainfall is insufficient. ing for different fertigation strategies under micro- System maintenance. Drip irrigation irrigation. Ag Water Mgt 74:219–42. Maas EV, Grattan SR. 1999. Crop yields as affected by salinity. In: Skaggs RW, van Schilfgaarde J (eds.). systems should be designed for a high Hanson BR, Hutmacher RB, May DM. 2006. Drip Agricultural Drainage. Agron Monograph 38. ASA, uniformity of applied water, and should irrigation of tomato and cotton under shallow CSSA, SSA, Madison, WI. p 55–108. saline groundwater conditions. Irrig Drain Sys be properly maintained to prevent emit- 20:155–75. Schoups G, Hopmans JW, Young CA, et al. 2005. ter clogging. Sustainability of irrigated agriculture in the San Hanson BR, May DM. 2003. Drip irrigation in- Joaquin Valley, California. PNAS 102(43):15352–6. Drainage-water disposal creases tomato yields in salt-affected soil of San Joaquin Valley. Cal Ag 57:132–7. Šimunek̊ J, Šejna M, van Genuchten MTh. 1999. Can drip irrigation eliminate the The HYDRUS-2D Software Package for Simulating Hanson BR, May DM. 2004. Effect of subsurface Two-dimensional Movement of Water, Heat and need for expensive subsurface drainage drip irrigation on processing tomato yield, water Multiple Solutes in Variably Saturated Media. Ver. systems and drainage-water disposal table depth, soil salinity and profitability. Ag Water 2.0, IGWMD-TPS-53. International Groundwater Mgt 68:1–17. Modeling Center, Colorado School of Mines, methods? We believe the answer is yes, Golden, CO. 251 p. since no subsurface drainage systems Hanson BR, May DM. 2006. New crop coefficients developed for high-yield processing tomatoes. Cal Styles S, Oster JD, Bernaxconi P, et al. 1997. Dem- were used at our sites. Subsurface drip Ag 60:95–9. onstration of emerging technologies. In: Guitjens J, irrigation continues to be used at these Dudley L (eds.). Agroecosystems: Sources, Control sites along with many other fields along Hanson BR, May DM. 2007. The effect of drip line and Remediation. AAAS, Pacific Division, San Fran- placement on yield and quality of drip-irrigated cisco, CA. p 183–206. the San Joaquin Valley’s west side. processing tomatoes. Irrig Drain Sys 21:109–18. Drip irrigation resulted in little Wallender WW, Grimes DW, Henderson DW, et Hanson BR, May DM, Schwankl LJ. 2003. Effect al. 1979. Estimating the contribution of a perched change to the water table at these sites of irrigation frequency of subsurface drip irrigated water table to the seasonal evapotranspiration of (except at site BR, where overirrigation . Hort Technol 13(1):115–20. cotton. Agron J 71:1056–60. occurred), and the computer simula- Hanson BR, Šimunek̊ J, Hopmans JW. 2006. Westlands Water District. 2009. 2008 Crop Acre- tions revealed that drainage or percola- Numerical modeling of urea-ammonium-nitrate age Report. www.westlandswater.org. tion below the root zone would occur. The field data indicated that small ap-

136 CALIFORNIA AGRICULTURE • VOLUME 63, NUMBER 3