ARE CROP COEFFICIENTS FOR SDI DIFFERENT FROM THOSE FOR SPRINKLER IRRIGATION APPLICATION?

Subsurface drip irrigation (SDI) has become an important irrigation application method in the U.S. Southern High Plains where pan evaporation exceeds 2,400 mm per year. Early research comparing SDI with spray sprinklers showed that SDI was over-applied when scheduling irrigations using crop coefficients developed using sprinkler irrigation. Thus, crop coefficients developed using SDI may be smaller than those developed using sprinkler irrigation. Grain corn was grown for two years on large, precision weighing lysimeters at Bushland, Texas, with two lysimeters irrigated by SDI and two by mid-elevation spray (MESA) irrigation. Data used in this study were for fields irrigated to replenish soil water in the top 1.5 m of the soil to field capacity, as indicated by weekly neutron probe readings (100% replenishment). Crop coefficients developed for SDI (Kc_SDI) were compared with those developed for MESA (Kc_MESA) using ASCE standardized reference ET equations. The value of Kc_SDI ranged from 0.83 to 0.89 of Kc_MESA for the two years. Values of Kc_SDI remained consistently less than Kc_MESA even after maximum leaf area index was reached, indicating that considerable evaporative loss from the soil surface occurred with MESA irrigation even after full canopy cover. When we shortened the initial period after planting from 30 to 20 d and followed FAO 56 recommendations for surface drip irrigation under full-cover plastic mulch, we calculated basal Kc (Kcb) values (ETo basis) that were reasonably close to our Kc values for SDI for the crop development and early mid-season periods but were greater than our data for the later mid-season and late season periods. [ABSTRACT FROM AUTHOR]