Characteristics Of Evapotranspiration Based On Remote Sensing And Analysis Of Water Changes In Weihe River Basin, China

Under the water shortages, the regional evapotranspiration (ET) estimation is significant forthe rational development and utilization of water resources. The traditional ET estimationmethod can better reflect the physical mechanism of ET, but limited in the single-point scale.Combined with remote sensing, surface parameters are calculated, making the traditional methodfrom the single-point scale spread to the regional scale, becoming the focus in recent years.In view of the lack of necessary research on impact of ET, NDVI on water changes inWeihe River basin and the need of water resources management, using the Penman-Montiethequation, monthly and yearly ET in the periods with similar precipitation conditions wereestimated, and ET and water changes characteristics were analyzed, and impact of ET, vegetationchange and other factors on water changes were discussed. This was important to recognize therunoff reduction law, and to provide the foundation for improving the water environment andplanning the ecological environment. The main conclusions are as follows:(1) Analyzing precipitation characteristics and determining periods of ET estimationThe average annual precipitation is530.7mm, centralized from May to October. Since the1980s, the annual precipitation is declining and the interannual variability is getting smaller. Inthe spatial distribution, precipitation is reducing from southeast to northwest, and maximumoccurs in Huashan and Qinling Mountain and minimum in the Loess Plateau. Less than5%inthe difference of the annual precipitation, less than5%in the precipitation anomaly and close to1in the correlation coefficient of monthly precipitation sequence involve14periods with similarprecipitation conditions in1987,1993,1999,2001,2002and2009. The difference in the annualprecipitation and precipitation process is very small and the spatial distribution is close, and it isbasically similar runoff precondicion for analyzing the impact of ET on water changes.(2) Analyzing spatiotemporal law of ET and validationThe average ET is371.4mm in2009, accounting for75.3%of precipitation, mainly fromApril to October. High-value is located in the middle and lower reaches of Weihe and JingheRiver, and low-value in the upper reaches of Jinghe River and Qinling Mountains. From thesub-basin, ET is the highest in the main stream of Weihe River with obvious spatial difference.The spatial distribution of monthly ET is identical to yearly ET, but ET in Ziwuling region, Long Hill, Kongtongshan Mountain and lower reaches of Weihe River is relatively high in summer. ETaccounts for morn than70%of precipitation and shows an increasing trend to a degree in6-year.The highest value occurs in July or August and the lowest in December or January.The ET estimation result in2009is in accordance with observated ET with the eddycorrelation (EC) in Changwu site, but a litter lower in values. The estimated ET with model inWeihe River basin and sub-basin are lower than that calculated by the water balance method, andthe error in Huaxian station is the smallest, closest to the water balance calculation result.Monthly ET is consistent with MOD16ET and better consistent with the1:1line. From differentland cover types, the range of values in P-M ET is more centralized than MOD16ET. Thecorrelation coefficient between P-M ET and MOD16ET is relatively higher for different landcover types in2001, and better consistent with the1:1line. Compared to the estimation result ofthe complementary relationship model, the P-M estimation is lower in the annual scale andsmaller numerical range in the monthly scale.(3) Analyzing water change characteristics and factorsThe average annual runoff volume is27.53billion m3in Tongguan station from1981to2011with bimodal variation in monthly. Since the1980s, the water amount in Tongguan stationis an obvious decreasing trend overall, and the trend eased after2000. The average annual runoffvolume in6-year in Jinghe and Weihe basin is4.11billion m3with a decreasing trend,accounting for88.0%of the total water volume in Weihe River basin, and19.6%of Tongguanstation. The annual average runoff in Luohe River basin is relatively small, but showing anincreasing trend. The confluence area in Zhangjiashan and Xianyang station are similar, but thedifference in the water amount is large, accounting for22.9%and50.9%of Huaxian,respectively. Linjiacun-Xianyang interval is the main water production area, and above Beidao isthe sediment yield area in Xianyang station and Zhangjiashan and above Beidao are mainsediment source in Weihe River. Huaxian, Zhangjiashan, Xianyang, and Zhangjiashan/Xianyang-Huaxian interval shows a unimodal distribution in monthly, and peaks in Zhangjiashan/Xianyang-Huaxian interval appear in June, others in July.In similar precipitation condition, with the rise of NDVI and the improvement ofvegetation coverage, the regional ET increases and the water amount decreases, but ET is not adecisive factor for water changes. Through river runoff diversion and groundwater exploitation,human activities produce a significant impact on water changes.