This paper takes the non-irrigated plain areas in Xinjiang (covering a total area of 505944km²), where the natural vegetation draws water mainly from the shallow groundwater to satisfy its demands. In the study the distribution and coverage of natural vegetation in various river basins was defined by interpreting the TM satellite images taken in August 2000 with a scale of 1:100, 000, and the unsaturated strata and phreatic groundwater depth were determined through hydro-geological investigation based on working maps with a scale of 1: 250,000. Based on the unsaturated strata, shallow groundwater depth anddistribution and coverage rate of the natural vegetation and by using GIS techniques, the calculation units for this study were then divided, of which the phreatic groundwater evaporation parameters and vegetation conversion coefficient were then calculated by using data of phreatic groundwater evaporation and transpiration from the typical area

water balancing experimental stations, and, based on all of those data and parameters obtained, the groundwater use by the natural vegetation in the study areas was calculated as 63.7762×10⁸m³/a under the current conditions.

1 General Introduction to the Study Area

According to the latest statistical data, the total land area in Xinjiang is 166.43×10⁴km², including mountainous area of 71.9704×10⁴km² (accounting for 43.24 %), plain area of 50.5944×10⁴km² (accounting for 30.40%), and desert area of 0.5261×10⁴km² (accounting for 0.32%). Within the plain area, the area of the oases (calculated as the area circled by the boundary line of the irrigated areas) is 8.1271×10⁴km² (accounting for 16.06% of the total plain area), natural vegetation (forest, grassland) in the non-irrigated area is 12.4311×10⁴km²（taking up 24.57% of the total plain area）, bare land area ( area with vegetation coverage rate <5%) is 30.0361×10⁴km² (accounting for 59.37% of the total plain area）.The plain area in Xinjiang is located in the arid area, where ecological water supply for the artificial oases comes mainly from irrigation, while natural vegetation in the non-irrigated areas (natural forest and grass) draws water mainly from the shallow groundwater to satisfy its demands.

2 Calculation Formula

In the non-irrigated area, water use by the natural vegetation is just the phreatic groundwater evapo-transpiration, and it can be derived by multiplying the area of a certain type of vegetation under condition of a certain groundwater depth by the evaporation of the phreatic groundwater and by the vegetation conversion coefficient at that phreatic groundwater depth, e.g.:

W=ΣA_i·E_oi·C_i·K_i                                      (1)

Where: W— Groundwater use of natural vegetation in a study area; A_i— Area of a certain vegetation under certain shallow water depth; E_oi— Water surface evaporation in a certain calculation unit, expressed in data from E₆₀₁；C_i—phreatic groundwater evaporation coefficient of a certain calculation unit under bare land condition; K_i— Vegetation conversion coefficient, i.e., ratio of phreatic groundwater evaporation in area with vegetation to that in bare area with other conditions in such two types of areas being identical.

3 Steps of the Calculation

3.1 Definition of areas of forest and grassland in the non-irrigated areas of the river basins

TM satellite images taken in August 2000 with a scale of 1:100, 000 were processed to define the areas and coverage ratios of natural vegetation in the counties/cities of the river basins included in the study area, and an electronic map Distribution of Natural Vegetation in the Non-irrigated Plain Areas (1:250,000) was prepared. The total area of natural vegetation in the non-irrigated plain areas in Xinjiang is 124311km², as detailed in the following Table 1.

3.2 Investigation of unsaturated strata and shallow groundwater depth

Unsaturated strata and shallow groundwater depth are the major factors impacting on the phreatic groundwater evaporation in the bare land area, hence hydro-geological investigation was conducted in April through August 2003 based on working maps with scale of 1:250,000, and electronic maps Distribution of Unsaturated Strata (1:250, 000) and Zoning of Shallow Groundwater Depth (1:250,000) were produced. The unsaturated strata have 4 categories, such as loam, sandy loam, fine silt and sandy gravel, and shallow groundwater has four levels of depth ( ≤1m, 1～3m, 3～6m, and >6m).

3.3 Division of calculation units

Using GIS techniques and based on maps Distribution of Natural Vegetation in the Non-irrigated Plain Areas (1:250,000), Distribution of Unsaturated Strata (1:250, 000) and Zoning of Shallow Groundwater Depth (1:250,000), calculation units were divided and there are altogether 3927 calculation units. Unsaturated strata, shallow groundwater depth and natural vegetation are similar within the same calculation unit.

3.4 Definition of calculation parameters

(1) Evaporation Parameters of phreatic groundwater in bare land areas

For northern Xinjaing areas, the phreatic groundwater evaporation parameters were determined based on experimental data from the water balancing experimental stations in Changji, Wulabo and groundwater balancing experimental stations in Changji and North Fukang; for southern Xinjiang and eastern Xinjiang areas, they were decided based on data from Qunke, Turpan water balancing stations that belong to Xinjiang Geological and Mineral Bureau and from Shache groundwater balancing station. Table 2 shows details of those results. Phreatic groundwater evaporation coefficient of the calculation units in the river basins are obtained according to the water surface evaporation (data of E₆₀₁) and groundwater depth in the river basins (such coefficients are omitted in this paper).

Table 1    Distribution of natural vegetation in the non-irrigated plain areas in Xinjiang   Unit：km²

Table 2 Evaporation Coefficient (C ) of Shallow Groundwater

          (2) Vegetation conversion coefficient ( Ki)

               The vegetation conversion coefficient (Ki) were determined according to experimental data from Changji groundwater balancing experimental station of Xinjiang Geological and Mineral Bureau, North Fukang groundwater balancing experimental station of Xinjiang Water Resources Bureau, and from Yumenzhen experimental station located in Hexi Corridor of Gansu Province. The results are shown in Table 3.

3.5 Calculation of groundwater uses by natural vegetation in the non-irrigated areas
For each of the calculation units, the above-mentioned Formula 1 was used in calculating groundwater use and then the total groundwater use by natural vegetation in the study area is obtained as 63.7762×108m3/a by aggregating results of the river basins, as shown in Table 4.

Table 3 Vegetation Conversion Coefficient of Phreatic Groundwater Evaporation

Table 4 Groundwater Uses by Natural Vegetation in the Non-irrigated Plain Areas in the River Basins