Investigation of land use change effect on flow discharge using HEC-HMS model (Case study: Leaksha sub- basing of Neka)

Recent research results have shown that many factors such as population growth, extensive land use change, and natural resources destruction caused erosion, soil degradation and also flood. Therefore, hydrologic impacts of vegetation cover management and land use changes on minimum discharges variation, runoff depth, evapotranspiration, soil moisture and a severity of improper exploitation of land and, inappropriate use of land and the peak flow was indicated as the results of environmental changes (Sika et al, 2003). Therefore, a better understanding and assessment of land use change impacts on a watershed hydrologic process is of great importance for predicting flood potential and the mitigation of hazard, and has become a crucial issue for planning, management, and sustainable development of the watershed (Ali et al, 2011). Estimation of flow discharge is one of the important keys for planning and optimal management of water resources at watershed scale such as Laksha catchment in Mazandaran province in the northern part of Iran.
The study area is located between 36º 46' to 36 º 62' N and 53º 30' to 53º 58' E in the southern and the distance about 8 kilometers of Neka city. The maximum and minimum altitudes of this watershed are 1288 and 73 meters, respectively. The annual precipitation is 640 millimeter according to Paeen Zarandin station for 28 years duration. The annual relative humidity and temperature are 77% and 18Co, respectively. From a map at the scale of 1:25000, the geometric corrections and DEM was extracted which was used to remote sensing data corrections. For the geological data and soil maps, the same scale of 1:50000 was used with sources of the Iranian Geological Survey and Soil and Waters Conservation Organization. The extracted DEM map was produced from the mentioned topographic map with manipulated pixels in ArcGIS software. The dimension of the used DEM for this area was at 15×15 m2 for each pixel. In order to the image classification, firstly, some training area has selected to cover the watershed. Therefore, about 113 points from the study area were marked randomly with the optimum distribution. One of the effective factors in river flow regime variation is related to the basin physical characteristics, slope, vegetation type, geologic formation and land use changes. The land use changes of the study area were gained based on the satellite data of Landsat TM and the IRS-1D LISS-III images of 1984 and 2012 from Neka watershed as the main basin of Laksha sub-basin in advanced ArcGIS version 9.3 and remote sensing software of Envi 4.7. As pre-processing, different corrections were used such as geometric, atmospheric, enhancement and then random sampling was selected for the land classification of each image. The classification technique of maximum likelihood algorithm used to find the best classification method for the used images. The optimum band combination of 7, 4, and 1 from TM image with 0.86 percent of Kappa coefficient had 92.75 overall accuracy, and this combination in LISS-III with bands 2, 3, 4 resulted in 0.87 and 91.47, respectively.
The results of this research indicated that in the period of 28 years, the forest area was reduced to 1042 hectares (4.79%), the land without vegetation with 297 hectares (34.33%) and 745-hectare area (28.63%) of agricultural land also increased. For this purpose, the land use maps of 1984 and 2012 were integrated with hydrologic soil group maps to find CN in GIS environment. Then in HEC-HMS hydrologic model based on hydrograph curve number and SCS curve number infiltration unit hydrograph method and the surface area of the observed rainfall-runoff data were analyzed for calibration and validation, respectively. The results of the calibrated model parameters were optimized the curve number and lag-time of the watershed. The land use changes in this period influenced on peak flow rate and runoff in the basin outlet from 36.5 m3/sec and 78.4 mil/m3 1984 to 69.9 m3/sec and 119.55 mil/m3 in 2011. Based on the gained results from this study, the main land use change was related to the forest area. This land use change was caused due to the rural population growth and extensive agricultural activities such as tillage in slope direction. The forest area had the dominant land cover of the study area between different land use types. The existence agriculture area, especially the rain-fed land and dispread settlement area among the forest area has evidenced as human activities in landscape. Deforestation and land use changes were the main reason of runoff and flood occurrence in Laksha watershed. The sub-basin of number 1 measured with minimum channel gradient of 3.6% and maximum time of concentration of 201 minutes which expected to have lower peak flow and runoff, but the maximum rate of flow was due to the land use changes such as deforestation in study area based on remote sensing data of Landsat and IRS despite vegetation cover in agricultural type. This result confirmed the last studies of Farajzadeh and Fallah (2008), Saqafian and et al. (2006), Saadati and et al. (2006) and Chen and et al (2009). Finally, it could be concluded that the main reason for the runoff flow in Laksha watershed was related to the land use changes such as deforestation.