Quantifying Evapotranspiration (ET) for Wetlands in South Florida Ranchlands

Evapotranspiration (ET) is the largest component of the wetland water budget and its accurate quantification is important for quantifying the effects of rehydrating the wetlands located in ranchlands of south Florida. In this National Science Foundation Research Experience for Undergraduates (NSF-REU) Project, four ET models were evaluated for their accuracy and data requirement for quantifying ET from two isolated wetlands located in a ranch. The ET models were Penman-Monteith (PM), Penman-Combination (PC), Priestley-Taylor (PT), and the modified Turc (MT). Available weather and wetland water level data from these wetlands for the period from July to December 2009 were used to quantify ET using these four models. The effect of data availability on model predictions was evaluated by comparing the ET estimates obtained using daily and monthly water level data. Using surface water level data and a vegetation map for the two wetlands, time series of spatial extent of area under water, wetland vegetation, and pasture were determined in order to accurately estimate ET based on land cover. Predictions from ET models were used in conjunction with the rainfall, storage, and runoff from the wetlands to estimate wetland water balance and quantify the change in groundwater storage (ΔGW). Models were ranked based on their data needs and accuracy in quantifying ET and ΔGW for the two wetlands. Percent differences between the predicted ET from models ranged from 1-13%. Compared to the PM model, the PT was the most accurate model (Nash-Sutcliff E= 0.91) while the PC was the least accurate model (E= 0.70). Depending on the ET model used, the ΔGW estimates varied from 2 to 30%. The difference in ET estimates from daily versus monthly water level data was 15% indicating that daily values are needed to accurately quantify wetland ET using the models evaluated in this study. There exist considerable uncertainties in estimating ET which in turn can introduce errors in quantifying the groundwater component of the water balance. Accurate quantification of the effects of wetland water retention on surface and subsurface flows will depend on the accuracy of ET estimates.