EVALUATION OF THE IMPACT OF TWO COMMON AQUATIC PLANTS, TYPHA LATIFOLIA AND EICHHORNIA CRASSIPES, ON WATER LOSS FROM FRESHWATER PONDS

Evapotranspiration, as affected by growth changes of broad-leaf cattail (Typha latifolia) and water hyacinth (Eichhorina crassipes), was monitered along with meteorological conditions in order to assess the impact of these changes on pond-water loss rates and to develop predictive water-loss equations.; A range of plant population characteristics including leaf area, height above water, standing crop, and plant density was induced by maintaining two different fertilization regimes in 6 tanks containing cattail and 3 different regimes in 9 tanks containing water hyacinth. Water loss from tanks with plants was compared with evaporation from three tanks without plants and a class A evaporation pan. This permitted the measurement of water loss rates for divergent populations and provided for the association of descriptive plant variables with changing evapotranspiration rates. The range of plant population characteristics were similar to those of natural populations in the southeastern United States.; All plant treatments lost water at rates higher than control treatments. Evapotranspiration of cattail for the months of May through October was 1.45 to 1.76 times that of open water evaporation. Evapotranspiration of water hyacinth for the same period ranged from 1.50 to 2.15 times that of open water evaporation.; The importance of using a range of plant growth characteristics when developing predictive water loss equations was demonstrated; all predictive evapotranspiration models with r-square values greater than 0.90 incorporated one or more descriptive plant growth variables. These variables for the cattail included standing crop, plant density, leaf area, and height above the water. For the water hyacinth the variables were leaf area, height above water, and density.