| China is one of the 21 countries with the most severe water shortage in the world.The per capita fresh water amount is only 1/4 of the world’s per capita.The water shortage seriously restricts the sustainable development of China.At the same time,the demand of ecological environment for water is often ignored under the water resources management mode,which focused on social demand,and seriously restricts the sustainable development of fragile arid and karst areas in China.Water not only affects the process of vegetation restoration and rocky desertification control,but also has an important influence on the social and economic development in karst areas.Therefore,correct understanding on the available water resources in karst areas of southwest China would have great guiding significance in order to solve the problems of water resources shortage and water resources allocation,to prevent the further aggravation of the problems of regional ecological environment and rocky desertification,and to guarantee the development of regional economy.In order to provide a scientific reference for vegetation restoration and ecological environment water demand in the karst graben basin,this paper takes the Mengzi karst graben basin as the study area to estimate the ecological water demand of dominant vegetation and to discusses the water demand of vegetation and its temporal and spatial differentiation using the crop coefficient method recommended by FAO.The main research results are below:(1)The annual average value of potential evapotranspiration intensity of each meteorological station from large to small order is Mengzi(1284.8 mm)>Gejiu(1281.15mm)>Kaiyuan(1273.85 mm)>Yanshan(1084.0 mm)>Pingbian(1007.4 mm),almost all of which is stronger than the precipitation in the same period,and may aggravate the water shortage problem of Mengzi karst graben basin.At the same time,the potential evapotranspiration of the Mengzi karst graben basin is higher in the north region and lower in the south region,and higher in the west region and lower in the east region due to the influence of the three-dimensional climate in the mountains and different land uses.The annual potential evapotranspiration of Thatch,Lantana camara,Corn,Cypress and Apple is635.04 mm,798.83 mm,2740.10 mm,547.03 mm and 277.22 mm per unit area,respectively,which means that the crops in Mengzi karst graben basin consume more water and also need more water to maintain their operation.In addition,the potential evapotranspiration of the five meteorological stations fluctuated greatly from year to year,and their fluctuation trends were different.Among them,the potential evapotranspiration of Gejiu,Kaiyuan,Pingbian and Yanshan has shown an upward trend in the past 50 years,and only Mengzi station shows a downward trend,indicating that the potential evapotranspiration has largely regional characteristics.Meanwhile,the general upward trend of the potential evapotranspiration in the study area also aggravates the contradiction between water resources supply and demand.In terms of seasonal trends,the evapotranspiration of five meteorological stations are showed a single peak change with higher intensity in spring and summer and lower intensity in autumn and winter in Mengzi karst graben basin.In addition,the potential evapotranspiration in in Mengzi karst graben basin is consistent with the average temperature(℃),average wind speed(m/s)and total solar radiation(MJ/m~2)in the study area.(2)Under the influence of the distribution of cultivated land and the three-dimensional climate,the ecological water demand of the Mengzi karst graben basin decreases from the central part of the flat basin to the surrounding mountainous areas.The maximum values of ecological water demand appears in the Mengzi basin,Datun basin and Mengzi east mountain area,while the minimum value appears in the east and north of the graben basin.The total annual ecological water demand of Cypress,Lantana camara,Thatch,Corn and Apple are102.26×10~6 mm,157.90×10~6 mm,128.89×10~6mm,353.19×10~6 mm and 60.19×10~6 mm,respectively.Among them,the Apple has the largest daily water requirement,with an average of 1.38 mm in the dry season and 2.61 mm in the rainy season,while the Cypress has the least daily water requirement,with an average of 1.20mm in dry season and 2.17 mm in rainy season.At the same time,the water demand of farmland crops is generally greater than that of natural vegetation in Mengzi karst graben basin,which indicates that the selected farmland crops in this area consume more water and the natural vegetation growing is better adapted to the rocky desertification environment.In addition,the ecological water demand of vegetation in the study area also has obvious difference between dry and rainy season.Uunder the same land use the ecological water demand of each vegetation in rainy season is significantly higher than that in dry season,which indicates that the main water consumption period of dominant vegetation in Mengzi karst graben basin is mainly from June to September,matching the precipitation regular in the study area(3)More than 80%of the meteoric precipitation in the Mengzi karst graben basin is considered as effective precipitation and falls in the rainy season.Under the influence of the basin-mountain coupling topography the effective precipitation in this region increases from the central and northern part to the periphery,showing a“U”shaped distribution area,with the lowest value in the north and the maximum value in the west.Thus,it provides a reference for the spatial and temporal regulation of regional water resources.Moreover,the precipitation in Mengzi Karst grabne basin can satisfy the normal growth of most vegetation,which indicates that the dominant vegetation in this region has a high matching degree with the water resources in this region.However,due to the distinctive characteristics of the region’s dry and rainy climate,vegetation growth in the dry season requires additional water recharge(The total amount of supply of Cypress,Lantana camara,Thatch,Corn and Apple are 2.32×10~6 mm,9.83×10~6 mm,10.49×10~6 mm,12.24×10~6 mm and 7.29×10~6 mm,respectively.).The surplus water in the rainy season can alleviate the vegetation water shortage to a certain extent,indicating the importance of developing rainwater harvesting agriculture and implementing the spatial and temporal allocation of water resources in this region.In general,in order to realize ecological protection and economic development of Mengzi karst graben basin,the first thing is to solve the problem of water resources by means of increasing resources and reducing expenditure.In terms of flow reduction,the first is to appropriately increase ecological water consumption and save agricultural water consumption.The key is to upgrade the seepage control and irrigation technology of water conveyance projects and to take the sedimentary basins as the main reconstruction areas.The second is to save water and to change the traditional idea.It is necessary to increase the total amount of water resources through reasonably increasing exploitation of groundwater in this area.In addition,according to the spatio-temporal characteristics of precipitation in this area,we should build water conservancy facilities to store or/and allocate water and develop rain-collecting agriculture. |
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