| Agriculture and grassland ecological region of loess plateau,which located in the middle of the Loess Plateau,is facing a fragile ecological environment of harsh climate,water resources scarcity and uneven distribution.Climate warming and drying has a serious impact on the ecological environment of arid and semi-arid areas.It is helpful for the water resources management to quantify the change of regional surface dry and wet and the water demand of ecological environment temporally and spatially base on ecological regionalization.In this paper.,base on temperature vegetation drought index(TVDI)and Penman-Monteith model to constructe the surface dry/wet evaluation method and the ecological water demand estimation model,the temporal and spatial variations of the surface dry/wet conditions and ecological water demands of agricultural and grassland ecological region in the Loess Plateau during 2000-2015 are analysed,the relationships between surface dry/wet change and ecological water demand are quantified,and the influences of some related factors on the two relations are discussed.The main conclusions are as follows:(1)There is a significant spatial difference and regular change with Seasonal replacement between the vegetation index and land surface temperature of study area.The inter annual fluctuation of vegetation index shows a steady upward trend but not obvious,while the surface temperature shows a decreasing trend.(2)The surface dry/wet conditions in study area have significant spatial and temporal heterogeneity and Volatility.On the whole,the drought condition of study area has eased in recent years,but it's still facing problems of water shortage and large area's light drought The time series of TVDI shows a downward trend and a three years dry wet cycle.The annual variation of TVDI shows two high values,May-June and January-February,which means that it's prone to drought in spring,summer and winter in study area.TVDI increases from high altitude and high vegetation coverage area to low elevation and low vegetation coverage area.The western agricultural ecological zone,the Liupan mountain ecological zone and the Lvliang mountain ecological zone are mainly humid areas,while the center areas of the Shaanbei-Jinxi ecological zone are the main arid areas.TVDI seasonal variation of each ecological region characterized by the fluctuation range in spring and summer is small,autumn next,bigger change in winter.(3)The ecological water demand has a significant spatial heterogeneity and seasonality.The annual average ecological water demand is lower than other areas with a not obvious annual fluctuation.The average ecological water demand is about 237.28mm/km-2·a-1.The annual variation of ecological water demand has one peaks value,July-August.Ecological water demand gradually reduces from forest ecosystem to grassland and farmland ecosystem.The high value of ecological water demand occurs in the southeast constituted by Lvliang Mountain ecological zone,the middle Shaanxi ecological region and the Liupan Mountain ecological region.The ecological water requirements in the northern region constituted by the Northern Shaanxi and Shanxi ecological region,and the north area of the southeast ecological region are low.The ecological water demand of each ecological zone has its own fluctuation range,and its inter-annual variation shows a upward trend with highly consistent.The amount of ecological water requirement is the largest in summer and the lowest in winter.The ecological water demand in summer is obviously increasing.The inter-annual of ecological water demands float slightly in spring,autumn and winter and the differences are not significant.(4)The correlation between TVDI and ecological water demand is mainly negative correlation.In different seasons,the impact of drought-wet change on ecological water demand is different.The negative correlation between TVDI and ecological water demand is 68.58%,and the negative positive correlation gradually shifte from the temperate grasslands and forests in the northwest to the northern agricultural and pastoral areas from the spring to the winter.(5)Dry/wet grade and ecological water demand shows a negative correlation.There are significant different relationships between TVDI and ecological water demand in different drought-wet grades.The highest ecological water demand appears in the wet state,the lowest appeares in the case of water shortage.Under the conditions of wet,semi humid and normal,the ecological water demand decreases significantly along with the decrease of moisture content.Under the conditions of light drought and water shortage,the ecological water requirement is the lowest,and it decreases first and then increases along with the increase of drought degree.Under severe drought condition,the ecological water requirement decreases along with the increase of drought severity.(6)The partial correlation shows that the relation between ecological water demand and vegetation index is more significant than ecological water demand and surface temperature.Altitudes affect the surface drought-wet grade and variation range which makes the ecological water demand vary in different altitudes.In this paper,the estimation model of ecological water demand is improved,using remote sensing method to achieve ecological water demand assessment in a large area.The estimation and analysis of ecological water demand for different time scales in long time series are carried out.Combined with the temperature vegetation drought index to explore the relationship between the change of surface dry/wet and ecological water demand,so that the study makes the relationship between the surface dry and wet state and ecological environment changes more visual and quantitative. |
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