Submitted to: Agricultural Water Management
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/25/2008
Publication Date: 1/1/2009
Citation: DeTar, W.R. 2009. Crop Coefficients and Water Use for Cowpea in the San Joaquin Valley of California. Agricultural Water Management. 96(1):53-66. Interpretive Summary: Water is becoming a scarce and expensive resource and should be used efficiently. To properly plan and manage irrigation systems it is important to know how much water the plants actually use. The goal here with cowpea was to develop a means for calculating water use. A very efficient drip irrigation system was used to apply water daily to a two-acre field of sandy soil. Soil moisture was closely monitored for two seasons. We employed a relatively new method, developed for cotton, which uses the change in soil moisture to predict water use. The cowpea used water at a rate, which was about 11% higher than for cotton, but the total normal seasonal use was about the same-- near 26 inches. Yields were high the first season (2005) at 5320 lbs per acre, with a water use of 25.2 inches. For the second season (2007), the yield was 4170 lbs per acre with 24.4 inches of water. This information can be used to establish the proper amount of water to apply to cowpea.
Technical Abstract: To improve irrigation planning and management, an experiment was set up using a modified soil water balance to determine the crop coefficients and water use for cowpea in an area with a semi-arid climate and sandy soil. A 0.8-ha field was irrigated daily with a very efficient subsurface drip irrigation system and the soil moisture was closely monitored for two full seasons. Crop coefficients were developed to go with three types of reference evapotranspiration: 1) class A pan evaporation, 2) Penman-Monteith (P-M), and 3) a modified Penman from the California Irrigation Management and Information System (CIMIS). A face-to-face double sigmoidal function was used to relate the crop coefficients to growing degree days (GDD) and days after planting (DAP). The results showed that the use of DAP worked better for early-season data, but there was little difference between GDD and DAP for full-season data. The two-year averages for the mid-season values for the crop coefficients to be used with the pan, CIMIS, and P-M were 0.986, 1.211, and 1.223, respectively. The pan data had the least variability within seasons and from season to season. Normal water use for cowpea in this region was determined on a weekly, monthly and seasonal basis using the pan coefficients developed here, along with the long-term average values for local pan evaporation. One of the main findings from this work was that normal seasonal water use for cowpea in this area is 669 mm for a typical planting date. Yields were high the first season (2005) at 5966 kg/ha with a waster use efficiency (WUE) of 0.93 kg/m3. For the second season (2007) the yield was 4674 kg/ha with a WUE of 0.75 kg/m3.