Application of the Priestley-Taylor evaporation model to assess the influence..

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The influence of soil moisture on evaporation from a 6-m grass-covered lysimeter and from Class A pans was assessed for one summer using the α-parameter of the Priestley-Taylor evaporation model appropriate for the individual surfaces computed on a daily basis. Net radiation over the pan was estimated from above-grass measurements using a correlation established between the two, using measurements made in the previous two summers. Changes in heat storage of the water were considered in the derivation of α for the pan. A unique relationship for the particular conditions of the site was determined between the α for the lysimeter and soil moisture, approaching 1.29 at soil moisture near field capacity, but decreasing to as low as 0.5 for dry soil. The corresponding relationship for the pan showed more scatter, but this was improved by using 5-day running means of evaporation and stratifying the data in terms of wind speed to yield a family of curves. Values for α at wet soil conditions varied from 1.07 for 100 km day−1 wind run to 1.17 for 250 km day−1 wind run. For each curve, values of α increased by about 20%; as the soil dried. The relationships may be used to reduce observed Class A pan evaporation to equivalent values for wet-soil conditions and to estimate near-surface soil moisture and actual evapotranspiration for this particular site. Extension of the technique to other areas requires derivation of similar relationships appropriate for those other locations

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