Influence of Natural Water Hydrochemistry in the Vakhsh River Basin on the Water–Salt Regime of the Irrigated Agricultural Land

The irrigation of soil is an important measure for achieving the desired agricultural productivity, especially in regions with deficit of precipitations. The presence of sodium in the irrigation water can exert an adverse influence on the soil structure by reducing the rate at which water reaches the roots of plants. The Vakhsh river basin with more than 220 000 hectares of the irrigated land area is among the largest river basins in Tajikistan, which contains irrigation channels and a collector–drainage system with lengths of about 5000 to 6000 kilometers, respectively. The Vakhsh River merging with the Pyanj River to form the transboundary Amudarya River, which is the largest river in Central Asia, is the main river of the Republic of Tajikistan. The Vakhsh River Basin (VRB) is among the largest river basins (39 100 km²) in the highest part of Central Asia. In the VRB territory, 1213 million m³/year of the waste and collector–drainage waters are formed, of which 4 million m³/year are used for irrigation and 1209.1 million m³/year flow into natural surface water bodies. Therefore, the issue of water quality in the river basin is an urgent one. This study is aimed at analyzing the chemical composition of the Vakhsh River and its main tributaries and determining the degree of their applicability for irrigation of agricultural lands. It has been established that the Vakhsh River and its tributaries meet the requirements for irrigation water when regarding the main parameters, namely, the fractions of adsorbed, soluble, and exchangeable sodium and magnesium. It has been shown that the chemical composition of rivers is primarily formed because of weathering the rocks and washing out the skin of basins.