Study On Crop Water Requirement In The Middle Reaches Of Heihe River Basin Based On Sebs Model

Crop water requirement is very important for water cycle, and is a determinant for the estimation of water and heat transfer in the Soil-Plant-Atmosphere Continuum (SPAC). Quantitative estimation of crop water requirement is the base of appraising terrestrial NPP, regional water consumption, soil water transport, crop production, and land use/land planning. Especially, quantitative estimation of crop water requirement is important for the study on global or regional climate change.This study aims to simulate crop water requrement of the year2009by means of quantitative remote-sensing methods, the study ares covers the middle reach of Heihe River basin in northwest China. Due to the, for there are very comples terrains, various climate zones and diversiform land use, it is difficult to estimating crop water requrement at such spatial extent. In order to achieving this research purpose, the first step is simulation and analysis of crop water requierment, and secondly, sensitivity analyses of each parameters. This study lays particular emphasis analyzing on temporal and spatial analysis of crop water requirement of the middle Heihe river and the uncertainty of relevant key parameters of remote-sesing models for simulation crop water requirement, trying to arasp and understand physics mechanism of remote-sensing estimation of large space-time scale crop water requirement. According to this purpose, we firstly collected and processed relevant meteorological data, remote-sensing images, soil and vegetation data and ground validation information of the year2009, and secondly, on the basis of the remote sensing model of SEBS, we used this method to estimate crop water requirement of several days about the year2009. And quantitatively analyzedspatial-temporal patterns of crop water requirement. we also conducted the sensitivity analysis of the influence factors of SEBS. The main progresses includes:(1) Evapotranspiration in different monthsThe average monthly evapotranspiration was56.55,72.37,89.10,73.54,72.81and38.38mm respecively in April to September of2009. The maximum evapotranspiration was obtained in June and the minimum in September, the pattern of evapotranspiration showing a increase-then-decrease trend. Evapotranspiration from April to September varied from86.06to641.64mm, the evapotranspiration at Jiuquan region (in northeast part of study area),at Gaotai, Linze, Zhangye, Shandan (in middle part) and at those areas near Heihe river are relatively higher, the evapotranspiration in northeast and north-western desert region significantly less than farmland. The arable land boundary is reflected in the distribution map of evapotranspiration.(2) Spatial and temporal distribution of crop water requirementThe crop water requirement of growing season in four growth stages in2009varied significantly. The crop water requirement in study area of each stage were53.61,226.38,72.32and80.77mm, respecively. Within initial stage, the water requirement is minimum and account for11.56%of the entire growing season, whereas within development stage, the water requirement reaches maximum and account for48.83%of entire growing season. The crop water requirement in mid-season stage and late-season stage account for15.60%and17.42%. The crop water requirement in different stage over the entire growing stage follows a decreasing order:development stage> late-season stage> mid-season stage> initial stage.The daily crop water requirement in different growing stage was1.29mm,2.89mm,2.83mm and1.92mm, respectively. The different daily crop water requirement in dicreasing order as follows:development stage> mid-season stage> late-season stage> initial stage.The results show that the trend of crop water requirement gradually increased from south to north part, and crop water requirement varied from88.34to632.17mm over the entire growing season, with a mean of463.63mm. The spatial pattern of Crop evapotranspiration varied significantly. The crop water requirement for Linze, Zhangye and Jiuquan was relatively higher and very closed to560mm, and for Gaotai, Shandan and Minle, the water requirement was529.58,510.33and491.24mm, respectively.(3) Comparison of different methods for calculation of crop water requirementThe crop water requirement was calculated from PM-Kc method was greater than the the SEBS model simulated results in different growing stages. The daily average difference of crop water requirement for initial stage, the development stage, mid-season stage and late-season stage was0.03mm,0.43mm,1.24mm,0.62mm.(4) Input parameter sensitivity analysis of SEBS modelThe sensitivity analysis shows that daily evapotranspiration changed less than1%with the10%variation of average daily temperature, sunshine hours and air humidity. For the variation of wind speed, normalized difference vegetation index, surface emissivity and surface albedo, the daily evapotranspiration varied less than5%. The daily evapotranspiration varied larger than20%for the variation of ground coverage, the surface pressure, the reference height of the pressure and land surface temperature. The sensitivity analysis shows that the daily evapotranspiration varied acoording to the variation of each parameters as (from most sensitive to least sensitive):land surface temperature> reference height of the pressure> surface pressure> the percent of ground coverage> wind speed> surface emissivity surface albedo> normalized difference vegetation index> sunshine hours> air humidity> average daily temperatures.