Changes in Areal Potential Evapotranspiration by Land Cover Changes

A Case Study in Tama Hills

Abstract

Water balance on ground surface has been modified under the influence of changes in land covers. Potential evapotranspiration (Ep), which is one of the principal elements in a water balance equation, is calculated based on the method developped by Kotoda (1986). The changes in Ep are evaluated on the basis of bi-temporal LANDSAT images.
The study area of 3767 km' includes western part of Tokyo Metropolitan area, Tama hills and Sagamihara upland. In Tama hills, many housing developments are distributed along several railroad lines. Calculated area is divided into 120×120 meshes, and Ep is evaluated for each mesh.
Two LANDSAT MSS images, which were taken at Dec/14/1972 and Jan/23/1985, are used for land cover classification after geometric correction. Changes in land cover during 12 years are obtained for each mesh. Land cover mesh maps clearly show the enlargement of urban and residential areas, and the reduction of forested areas.
Albedo (p) and the soil heat flux constant (Cr) had increased by 1.3% and 2.1% respectively, and the net radiation had decreased by 45 MJm^-2 between 1972 and 1985. Annual Ep's for 1972 and 1985 are 592.6 mm/year and 573.9 mm/year respectively, and the change is -19 mm/year on the average. The decrease in Ep is large in Tama hill region, where the increase in residencial area, and the decrease in forested area are large.
The relationship among 4Ep (mm/year), 4p (%) and 4Cr (%) is obtained by multiple regression analysis as follows,
ΔEp=1.1-10.3Δρ-3.8ΔCr.
The equation shows that the increases in p and Cr lead to the decreases of Ep quantitatively.