Wind. The greater the wind speed or the daily wind distance, the more water • is transferred to the atmosphere by the process of convection. (Physiological Factors) Specific Crop. Differences in plant physiology can limit transpiration and • differences in reflectivity and roughness can limit evaporation. Stage of Plant Growth. This relates directly to the nature and amount of • foliage. Nature of the soil. As soil water content decreases, evaporation decreases. • Darker soils absorb more solar energy, resulting in more evapotranspiration. (Geographical Factors) Water Quality. The presence of salts in water causes a reduction in • evaporation. Depth to Water Table. The higher the water table, the greater the volume of • pores that can conduct water to the soil surface and the greater the rate of evaporation. Elevation. Elevation influences temperature and atmospheric pressure. • Methods of Measuring Evapotranspiration The methods used for measuring evapotranspiration fall into four classes: 1. Theoretical methods based on the physics or vapor movement (Not used for practical applications) 2. Empirical methods based on temperature, radiation, relative humidity, and other meteorological parameters (Used for long term predictions) Theoretical methods based on the energy balance (Used for short term predictions) Analogy from open water containers (Used for both long and short term predictions)
• Wind. The greater the wind speed or the daily wind distance, the more water is transferred to the atmosphere by the process of convection. (Physiological Factors) • Specific Crop. Differences in plant physiology can limit transpiration and differences in reflectivity and roughness can limit evaporation. • Stage of Plant Growth. This relates directly to the nature and amount of foliage. • Nature of the soil. As soil water content decreases, evaporation decreases. Darker soils absorb more solar energy, resulting in more evapotranspiration. (Geographical Factors) • Water Quality. The presence of salts in water causes a reduction in evaporation. • Depth to Water Table. The higher the water table, the greater the volume of pores that can conduct water to the soil surface and the greater the rate of evaporation. • Elevation. Elevation influences temperature and atmospheric pressure. Methods of Measuring Evapotranspiration The methods used for measuring evapotranspiration fall into four classes: 1. Theoretical methods based on the physics or vapor movement (Not used for practical applications) 2. Empirical methods based on temperature, radiation, relative humidity, and other meteorological parameters (Used for long term predictions) Theoretical methods based on the energy balance (Used for short term predictions) Analogy from open water containers (Used for both long and short term predictions)
Evap.docx