Investigation of plant-production potential on Kashan sand- dune systems using water balance model.

Lack of sufficient water is one the important feathers of desert regions and it is very important from different aspects. Considerable area of Iran's sand dunes (about 4.5 million hectares) are sensitive to wind erosion. There have been many sandstorms in the study area that have caused huge losses of arable soil and threatened the vegetated area and living environment of local people. Water is the most important restrictive factor for vegetation growth in arid and desert areas particularly in sand dune systems for development of vegetation cover such as rangeland, forest and agronomy. Thus it is necessary to study water balance on sand dunes. A conceptual water balance model for the one of the sand dune of Kashan area namely Rig Boland was constructed based on the principles of hydrologic cycles of water. Inputs include rainfall, any flow from groundwater capillary rise or water vapor due to condensation process and watering such as irrigation. Outputs are the losses of water out of the sand profile by percolation to water table by, evaporation from sand surface areas, evapotranspiration from grasses, bushes and trees and the water intercepted by the tree canopy and re-evaporated before it can reach the ground. Storage change is the difference in the amount of water in the dune system. This water can be held in the dunes as water holding capacity of the dune sand and, in the soil moisture zone where plant roots extract water. Changes in storage in the saturated sand are visible as changes in the water table level. Moisture content of the sand dune for the selected sites both for bare and vegetal dune were measured monthly using TDR device. Meteorological data such as daily precipitation, temperature, potential evaporation from adjacent meteorological station were collected and evaluated with monthly recorded moisture content using general water balance equation. Results showed that bare sand dunes without any vegetation could be classified as semi infiltrative that means, deep percolation is negative for the consecutive months of khordad (end of Spring) up to Aban (in Mid Autumn) periods, therefore rainfall can not recharge water table but in winter time up to mid summer the water balance for percolation is positive and rainfall can replenish underground water. In vegetal area posses native shrubs in GhasemAbad, percolation of water is negative mostly for the whole year except in late winter up to mid spring. It is concluded that in a natural condition of desert ecosystem it appears that in the investigated are natural rainfall is sufficient to establish xerophytes shrub and trees such as Haloxylon and other native shrubs. It seems that, precipitation is the only major source of water requirement for dune plants and the role of under ground waters in the form of vapor condensation is rarely important in this area due to existence of a hard pan between 4 to 5 meters deep from sand surface.