Evaluation of sugarcane evapotranspiration from water table data in the everglades agricultural area

The limited fresh water supply in south Florida has emerged as a major point of contention between the urban, environmental, and agricultural sectors. The mandated water quality improvement measures suggest the retention of water on farms (up to 20% of historical drainage pumping) could further impact the regional water supply. Accurate measures of evapotranspiration (ET) are vital in order to prudently plan for the equitable distribution of water. The daily changes in field water table depths were used to evaluate the actual evapotranspiration for sugarcane grown in the Histosols of the Everglades Agricultural Area (EAA) in south Florida. Water table recorders were installed in eight independently irrigated and drained plots, each 0.55 ha. Water table monitoring was conducted for a period of two years, 1990 and 1991, and the results were the average of these two years. The crop was plant and first ratoon. Each month, periods of time when no rainfall, irrigation, or drainage occurred were selected for use in ET determination. The time periods ranged from 5 to 19 days. Assuming that there were no seepage or deep percolation the daily declines in water tables, which exhibited clear diurnal patterns, were then used to determine the change in soil water losses during the selected time periods, the change in soil water storage was representative of actual sugarcane ET. Pan evaporation and crop growth were also monitored. The amount of water released from the soil during drainage was determined in laboratory columns. Five transparent PVC pipes, 12.1 cm in diameter were used to remove undisturbed 110 cm deep cores from the field. The columns of soil were saturated and drained to determine the drainable porosity of the soil. The water table data exhibited diurnal trends, with declines close to zero at midnight and maximum declines occurring between 2:00 and 3:00 pm. The data showed that minimum ET rates occurred during December through February (0.7-1.5 mm day^t-1 ). Maximum ET rates (4.5-4.6 mm day^t-1 ) occurred during June through September. Total ET was 106.2 cm per year. Using the actual field measured ET, crop coefficients (K_c) were calculated for sugarcane for the Penman, Thornthwaite, and Blaney-Criddle methods of determining potential evapotranspiration (ET_p).