Micrometeorological Model for Estimating Evaporation from a Bare Field in the Hetao Irrigation District in the Yellow River Basin

Abstract

In order to elucidate the water balance in the Hetao irrigation district (1.2 million ha) in the Yellow River basin, evaporation (E) from a bare field was estimated and a micrometeorological model of E was developed. Micrometeorological conditions above the bare field were measured to evaluate radiation balance and energy balance in the field from 16 September to 8 October 2003. Hourly E was estimated by the Bowen ratio - energy balance method. Total E from 16 September to 8 October was estimated as 34.3 mm, while rainfall was 36.1 mm during the period. Therefore, 95 % of water input to the field has been lost by E and the remaining 5% is supposed to have been stored in the groundwater in this period. This showed a typical water balance in a bare field in an arid region and E estimated by this method (EBowen) could be evaluated to be in good agreement with actual E. The applicability of three simple E models: E ratio, Penman Monteith (PM) and the bulk equation methods were compared by checking the merits of model parameters. The parameters, E ratio and evaporation resistances of PM and the bulk methods were calculated by EBowen and measured micrometeorological conditions. Both evaporation resistances could be related to soil water content in high correlations, while E ratio could be related to soil water content and other micrometeorological conditions. Therefore, E ratio should be parameterized not only by soil water content, but also other micrometeorological factors (such as temperature, humidity, wind speed and solar radiation, etc). The PM and/or the bulk methods coupled with the evaporation resistance models were better than E ratio because evaporation resistance which had only one parameter - soil water content, was useful.