| Socio-economic development with its internal requirement of water have facilitated contradictions not only between water supply and demand,but also among industries of human system and environment health in the precipitation-sparse and water-scarce Northwestern China(NC).Basically,formation of hydro-systems and distribuion of water resources were sourced and shaped by precipitation in the area.Thus,study on spatio-temporal variation of local precipitation across the whole could help leading better understanding and be of great significance to rational water utilization,ecological restoration and environmental sustainability in the area.Tropical Rainfall Measuring Mission derived precipitation(TRMM 3B42)provides opportunities for above studies due to its relatively high spatial resolution and large monitoring span.In this study,divisions of geographic-ecologic regions and mountain-plain systems were conducted.TRMM data were downscaled from 0.25°×0.25°to 250 m×250 m to gain precipitation distribution in higher spatial resolution.Then a comprehensive study was persued based on geo-statistics to analyze spatio-temporal patterns and variations of precipitation in NC.Following are what we have concluded.1.We defined in our study the boundary of NC with the 4 geographic-ecologic regions in it based on dataset illustraing heterogeneity of administrative units,natural resources and landscape distribution.The total area of the NC were determined in343.50×10~4 km~2,within it areal ratios of the 4 regions including arid regions of inner river basins(NWIR),the part of yellow river basin in the Tibetan Plateau(NWTP),the part of China's Loess Plateau(NWL)and the part of Inner Mongolia Plateau(IMP),from west to east,were orderly numbered 64.2%,13.5%,13.2%and 9.0%,respectively.Parameters derived from raster maps of altitude,slope and the Relative Hight Defference(RHD)were used to illustrate mountains and plains in NC.Results revealed that areal percentages of the two occupied 32.8%and 67.2%,respectively.2.Statistics at a time span of 64 years(1954-2017)based on station-observed series resulted into an overall slight decrease of precipitation in NC.M-K method tested into an increase-then-decrease trend during the period while Sen's slope statistics resulted into an overall decrease in regions of IMP and NWL,with decreasing ratioes from 0.0-3.3 mm/a.Opposite situations were found in regions of NWIR and NWTP,with increasing ratios from 0.01-2.2 mm/a.3.Scattering spots fittings presented a satisfactory applicability of TRMM precipitation in NC.Raster maps of the Normalized Defference Vegetation Index and the Normalized Defference Elevation Index(COSINE)were used for statistically downscaling the TRMM precipitation in mountain areas,while in plain areas,a method of simple downscaling was conducted for high resolution precipitation gaining.Mosaic approach was used for mapping precipitation distributions across NC during the time period from 2001-2016.Precision of downscaling was verified into satisfactory reliability.Index of R~2(Coefficient of Determination)and NSE(Nash–Sutcliffe Efficiency Coefficient)were calculated into acceptably minimum values of0.90 and 0.78,respectively.4.Obviously spatio-temporal heterogeniety of precipitation was found across the area of NC.Corresponding to geographic extending from South to North and from East to West,there presented a decrease then an increase trend of precipitation in space.Mountainous precipitation was overall higher than in plain area.The maximum amount of annual precipitation were found in mosoon influenced southeast part and the area of Western Tian Mountain of NC.While the minimum amount of annual precipitation were found in desertified areas north to the Hexi Corridor and desert area south to the Tian Mountain.There approximately presented an overall increase trend of precipitation form central-west to west and east parts of the area.5.Remarkable differences were found in precipitation distribution in the 4geographic-ecological regions of NC.Amount of regional precipitation were multi-yearly averaged and sorted into an order of NWL>IMP>NWTP>NWIR,from the maximum to the minimum.Variation analysis resulted into both increase and decrease in the 4 regions,although areal ratio of the former was higher than that of the latter.Relatively higher increasing ratios were found in areas of northeastern IMP,northern NWIR(the Junggar Basin)and the southern NWTP.While the relatively lower increasing ratioes were found mainly in desertified areas north to Hexi Corridor and the desert area south to the Tian Mountain,both located in NWIR.Increasing ratioes were sequentially compared into an order of IMP>NWL>NWTP>NWIR,going upward along with the altitudes in the NWIR,while in the other three regions,an inverse variation trend was found.The study could be a case example of data preparation and method exploration of remote sensing based precipitation in fields of rational water utilizaiton and allocation for the purpose of socio-economic-ecological restoration and environmental sustainability in the water-scarce Northwestern China. |
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