Crop coefficients and water use for cowpea in the San Joaquin Valley of California

To improve irrigation planning and management, a modified soil water balance method was used to determine the crop coefficients and water use for cowpea (Vigna unguiculata (L.) Walp.) in an area with a semi-arid climate. A sandy 0.8-ha field was irrigated with a subsurface drip irrigation system, and the soil moisture was closely monitored for two full seasons. The procedure used was one developed for cotton by DeTar [DeTar, W.R., 2004. Using a subsurface drip irrigation system to measure crop water use. Irrig. Sci. 23, 111-122]. Using a test and validate procedure, we first developed a double sigmoidal model to fit the data from the first season, and then we determined how well the data from the second season fit this model. One of the results of this procedure was that during the early part of the season, the crop coefficients were more closely related to days-after-planting (DAP) than to growing-degree-days (GDDs). For the full season, there was little difference in correlations for the various models using DAP and GDD. When the data from the two seasons were merged, the average value for the crop coefficient during the mid-season plateau was 0.986 for the coefficient used with pan evaporation, and it was 1.211 for the coefficient used with a modified Penman equation for ET(0) from the California Irrigation Management and Information System (CIMIS). For the Penman-Monteith (P-M) equation, the coefficient was 1.223. These coefficients are about 11% higher than for cotton in the same field with the same irrigation system. A model was developed for the merged data, and when it was combined with the normal weather data for this area, it was possible to predict normal water use on a weekly, monthly and seasonal basis. The normal seasonal water use for cowpea in this area was 669 mm. One of the main findings was that the water use by the cowpea was more closely correlated with pan evaporation than it was with the reference ET from CIMIS or P-M. Published by Elsevier B.V.