Study on Response of Evapotranspiration Consumption of Forest and Grass Vegetation to Natural Precipitation in Northwest Loess Plateau

In this paper, the evapotranspiration balance of forest and grass vegetation in the Loess Plateau of Northwest China in different regions was analyzed using 6 indexes in 3 categoriess, namely, evapotranspiration ratio (Ea/Q, Ep/Q), evapotranspiration difference (Q-EA, Q-EP), and actual (potential) water supply ratio (1-Ea/Q, 1-Ep/Q). It is used to objectively reflect the suitability of different types of vegetation in different periods of growth based on precipitation. In another words this suitability reflects the support capacity of natural rainfall to vegetation consumed water through evapotranspiration under the specific climate environment of the Loess Plateau. The results show that: (1) The actual evapotranspiration water consumption of all types of vegetation in this region increased significantly in the first three months of the growth period from April to June, resulting in a relatively high moisture dryness index of vegetation with an average k value of 0.44. The main reason was that natural precipitation was less at this stage, and the gradually rising temperature strengthened the transpiration of most vegetation. The forest was the most stressed. At the end of May and the beginning of June, with the increase of natural precipitation, the average k value of all types of vegetation began to decline. From July to September, due to the flood season in this region, the precipitation increased sharply, and the moisture dryness index was in the lowest range of the whole growth period, and the average k value varied between 0.26 and 0.30 with the lowest value was 0.26 at the end of August and the beginning of September. (2) It is obvious that the water stress of forest is higher than that of shrub and grassland. It is fully indicated that the difference of transpiration caused by the difference of vegetation types leads to the difference of actual evapotranspiration water consumption of different vegetation types.