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 agricultura l 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 tabl e depths were used to evaluate the actual evapotranspiration for sugar cane grown in the Histosols of the Everglades Agricultural Area (EAA) in south Florida. Water table recorders were installed in eight indepe ndently irrigated and drained plots, each 0.55 ha. Water table monitor ing was conducted for a period of two years, 1990 and 1991, and the re sults were the average of these two years. The crop was plant and firs t ratoon. Each month, periods of time when no rainfall, irrigation, or drainage occurred were selected for use in FT 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 exhibit ed clear diurnal patterns, were then used to determine the change in s oil water losses during the selected time periods, the change in soil water storage was representative of actual sugarcane ET. Pan evaporati on and crop growth were also monitored. The amount of water released f rom the soil during drainage was determined in laboratory columns. Fiv e transparent PVC pipes, 12.1 cm in diameter were used to remove undis turbed 110 cm deep cores from the field. The columns of soil were satu rated and drained to determine the drainable porosity of the soil. The water table data exhibited diurnal trends, with declines close to zer o at midnight and maximum declines occurring between 2:00 and 3:00 pm. The data showed that minimum ET rates occurred during December throug h February (0.7-1.5 mm day(-1)). Maximum ET rates (4.5-4.6 mm day(-1)) occurred during June through September. Total ET was 106.2 cm per yea r. Using the actual field measured ET, crop coefficients (K-c) were ca lculated for sugarcane for the Penman, Thornthwaite, and Blaney-Criddl e methods of determining potential evapotranspiration (ET(p)).