Micrometeorological Model for Estimating Evapotranspiration from an Irrigated Maize 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, evapotranspiration (ET) from an irrigated field of maize, which is one of the main crops in this district, was estimated and micrometeorological models of ET were developed. Micrometeorological conditions within and above the maize canopy were measured to estimate ET and to decide model parameters from 31 July to 14 September 2002 during the flowering and ripening stage. The plant height of the maize was 340 cm and LAI was 6.9 on 30 July. Daily ET was estimated by the Bowen ratio - energy balance method by integrating hourly ET. The total ET after irrigation, i.e., from 5 August to 14 September was estimated as 97.3 mm, while roughly 100 mm was irrigated on 4 August and rainfall was 8.6 mm during the period. Thus, 90 % of water input to the field was lost by ET and the remaining 10 % is assumed to have percolated and have been stored in the subsurface layer because of the lack of surface drainage in this period. This shows a typical water balance in an irrigated field in an arid region, and ET estimated by this method (ETB) could be assessed as being in good agreement with actual ET. The applicability of three simple ET models; ET ratio, Penman Monteith (PM) and the bulk equation methods were assessed. The parameters ET ratio and canopy resistances of PM and the bulk methods were calculated by ETB and measured micrometeorological conditions. Both canopy resistances could be highly related to solar radiation and soil water content, while no reasonable relationships were found for ET ratio. The PM and/or the bulk methods coupled with the canopy resistance models are expected to reproduce ET from the irrigated maize field successfully.