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Title: USING A SUBSURFACE DRIP IRRIGATION SYSTEM TO MEASURE CROP COEFFICIENTS AND WATER USE OF COWPEA (VIGNA UNGUICULATA).

Author
item DETAR, WILLIAM
item FUNK, HOWARD

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/1/2006
Publication Date: 3/1/2006
Citation: DeTar, W.R., Funk, H.A. 2006. Using a subsurface drip irrigation system to measure crop coefficients and water use of cowpea (vigna unguiculata). Meeting Proceedings. Calif. Dry Bean Advisory Board Progress Report. p. 45-49.

Interpretive Summary: Applying the proper amount of irrigation water at the proper time can be factors that affect the yield of cowpea. In this study water requirements were measured using a procedure that was developed and published for cotton, using a very efficient subsurface drip irrigation system. A 0.7-ha field on sandy loam soil at the UC Shafter Research and Extension Center, in Shafter, CA, has been in use for irrigation regime and water-use experiments since 1996. Water was applied daily to CB46 cowpeas, planted on 0.76 m row spacing. Soil moisture readings were taken twice a week in the plant row to a depth of 1.5 m at 24 sites in the field. The depth of water applied was adjusted twice a week with the goal of holding the total moisture constant. It was found that during mid-season, an average of 98.7% of pan evaporation and 125.3% of the CIMIS reference evapotranspiration were needed to hold the soil moisture constant. The cowpea used water at a rate that is 12 to 15% higher than that found for cotton. The total depth of water applied over the season 607 mm. An additional 38 mm was depleted from the soil. The yield was 5940 Kg/ha, which was nearly three times the yield of the furrow-irrigated cowpea on the same station, and almost double the average yield in the region.

Technical Abstract: Crop coefficients and water use by CB46 cowpea were measured using the same field and the same "slope" procedure developed and published for cotton, using a highly-efficient subsurface drip irrigation system on sandy soil at the University of California Shafter Research and Extension Center, near Shafter, CA. In 1996, extra-large diameter (22 mm) dripperlines were buried 27 cm below grade in every plant row, with a row spacing of 0.76 m and a row length of 100 m. Distribution uniformity (DU) was measured in the year 2000 and again in 2005, with the DU³95% in both cases. Water was applied on a daily basis. A neutron probe was used to measure the soil moisture in the plant row at 24 sites in the field, and this was done twice a week, down to a depth of 1.5 m. The crop coefficients for the mid-season plateau were found to be 0.987 and 1.253 for the pan evaporation and the CIMIS reference ETo, respectively. The same coefficients for cotton were 0.877 and 1.089, respectively, showing that the cowpea uses water at rate that is 12 to 15% higher than cotton. Full canopy cover was achieved by the crop, with a considerable increase (20 to 23%) in the crop coefficient as the canopy increased from 80% to 100% ground cover. The total depth of water used for the season was 645 mm. The yield was 5940 Kg/ha, compared to an average 2130 Kg/ha for furrow-irrigated cowpeas on the same station, and 3139 Kg/ha for the nearby region.