Seasonal Variations in the Heat and Water Balances for Nonvegetated Surfaces

Abstract

A model is presented for estimating the seasonal variations of evaporation, soil-water content, and soil temperature over nonvegetated land surfaces, especially in arid and semiarid regions. In the model, several types of soil are taken into consideration. Verification of the model has been achieved by comparing the observed and calculated results for the volcanic ash soil surface in Tsukuba and a sand dune field in Tottori. Using the routine data of 30 observatories including moist, semiarid, and arid regions at different elevations in China, the heat and water balances are estimated by the model. At a station in a semiarid region such as Lanzhou, the sensible heat flux is found to be considerably greater than the latent heat flux during the dry season. Both fluxes, however, have comparable magnitudes during the rainy season. The annual mean value of the soil-water content increases with depth, and the rate of increase grows larger as the amount of annual precipitation increases. But for an arid station such as in Turpan, the profile of annual mean value of soil-water content does not increase with depth, and the soil-water evaporated from the soil surface during the day and came back to the soil surface at night. At a station having a snowfall during the winter in Altay, the ground-surface temperature begins to rise just after the disappearance of the snow cover. Consequently, the sensible and latent heat fluxes increase abruptly from negative values to positive values. Soil-water content becomes abundant due to the melted snow. It is found that the annual amount of evaporation depends on the annual amount of precipitation. That is, in the arid region, it is proportional to the annual amount of precipitation. On the other hand, in the humid region, it tends to have a limited upper value, determined as functions of the potential evaporation and soil type. The water retentivity of loamy soil is better than that for sandy soil while the water permeability of sand is the best. These differences yield the result that the loamy soil has larger annual evaporation than the sandy soil. In arid and semiarid regions, a greater portion of rainwater is lost to evaporation within a few days after rainfall, so water resources become scarce. For a region having a snow cover during the winter, however, a considerable amount of melted water formed in the spring, resulting in a remarkable contribution to water resources.