Influence of Vegetation Cover on Regional Evapotranspiration in Semi-Arid Watersheds in Northwest China

Abstract

Spatiotemporal variability in these factors determines the variability of ET across large areas.This variability increases as the complexity of the underlying terrain and vegetation cover intensifies.Back and forth exchange of water vapor and liquid water from oases at the base of the Qilian Mountains (Northwest China) and from the Qilian Mountains to oases as surface and shallow subsurface flow has been previously shown to be a potentially significant mechanism in the long term maintenance of oases in westcentral Gansu, NW China [7].In general, direct precipitation (rain + snow) to the foothills and oases at the base of the Qilian Mountains is inadequate to maintain vegetation in these areas.Maintenance of oases vegetation is shown to be dependent on the surface water flowing from the higher regions of the mountain range, where direct precipitation and snowmelt are greatest.Supply of atmospheric moisture that leads to the formation of precipitation in the mountains is sustained by seasonal ET at the base of the mountain range.Influence of vegetation and landcover on local and regional climate is well documented in the scientific literature [7][8][9].Changes in landcover result in changes in surface albedo, surface roughness, leaf area index (LAI), stomatal conductance, rooting depth, and soil texture and structure [10][11][12].Changes in surface vegetation have been known to impact i.the partitioning of net radiative energy into sensible and latent heat fluxes, andii. the formation of convective rainfall by affecting the state of the convective boundary layer [13].Agricultural landuse affects climate by modifying the physiological attributes (e.g., canopy conductance) of the land [14]. AET-determination methods Point measurementsSuitability of AET-measurement methods depend on the reasons for assessment and on the spatiotemporal dimensions of the problem.Measurement of AET is mainly done indirectly by measuring the effect of AET on the water and energy balance.Rose and Sharma [15] categorized AET-measurement methods into three broad categories, i.e., i. hydrological approaches, based on the residual of the water-budget equation; ii.micrometeorological approaches, including the Bowen ratio [16], aerodynamic, and eddy covariance-based methods [17], and iii.plant-physiological approaches at the scale of individual plants or groups of plants by collecting sap-flow or chamber measurements.