Document Type : Research Article
Authors
Department of Range and Watershed Management, College of Natural Resources and Environment, Ferdowsi University of Mashhad, Iran
Abstract
Introduction: Over the past decades, millions of hectares of high-quality lands have been converted to other uses and low-yielding agriculture, which have had some unpleasant consequences for watershed hydrology. Analysis of hydrological responses of different basins to land use change has shown that correct land use balances the hydrological status of the basin, so that land use and the type and density of vegetation play an effective role in permeability and runoff reduction by changing humidity, soil organic materials and soil structure. Dimensions of hydrological effects of land capability in Chehel chay watershed in Golestan province, which is affected by land use change and deforestation, are more important. Therefore, this study seeks to investigate different scenarios of land use change and its effect on the hydrological status of the basin.
Materials and Methods: The J2000 hydrology model was used to simulate the hydrology of the basin. To better investigate the spatial and temporal variations of the hydrological parameters of the study area, it is divided into 2013 hydrological response units. After calibrating the J2000 hydrological model, the model was fed by rainfall data (1992-2014) and land use potential.
Results and Discussion: To evaluate the performance of the model, the dataset obtained in the time period of 2002-2014 was used for selection simulation and the first nine-years was considered as the calibration period and the remaining was considered as the validation period. The R2 of 0.67 and 0.55, and NAS coefficients of 0.83 and 0.76 were found in the calibration and validation periods, respectively. According to the ranking of Moriasi et al., the model efficiency is "good" and can be used in the present study. Several studies with similar observational data have reported similar results. The results showed that in summer and in May and June, the emptiest space in LPS soil pores is 3.07 and 3.21%, respectively. Increasing the consumption of MPS soil pores has also increased, and from 0.5 to 1.69% of the empty pores in the average soil pores has increased in these months. Therefore, increasing water storage in LPS pores in the months of May to June, surface runoff (RD1) decreased within the range of 6.28-26.38%, and the range of subsurface runoff (RD2) reduction was 4.41-8.41%. The amount of water percolation into groundwater aquifers was positive, and the highest infiltration into groundwater ranged from 0.83 to 1.72% for fast section groundwater (RG1), and from 0.48 to 0.52% for groundwater. Large pores do not hold much water, and water is transferred vertically to medium pores under gravity. When medium pores are saturated with water, water does not penetrate into these pores and remains in large pores and moves horizontally, increasing the subsurface flow. The results indicate that deforestation in order to expand agricultural lands and inappropriate use of the lands are the most important problems. Moreover, population growth has exacerbated the condition, necessitating proper land use management and planning. The scholars have also stated that proper land use has important effects on the water balance of watersheds.
Conclusion: In this study, the hydrological effects of land uses on the hydrological situation in Chehel chay watershed have been evaluated by simulations of the hydrological model. Our results reveal that the unplanned land use changes, land clearing, and expansion of agricultural lands have intensified the hydrological situation of the basin. The peak discharge of surface and subsurface runoff in hydrological response units decreased and the rate of water infiltration into soil and groundwater increased. Reduction of surface and subsurface runoff has also decreased the discharge in the basin outlet.
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