Modeling the effects of land use changes on flood hydrograph (A case study: Ja’farabad watershed, Golestan Province)

Background and objectives:Land-use and land-cover are considered as determining factors in studies of water resources, erosion, and sediment yield watershed (32). Computerized simulation of land-use change effects on the watershed processes have been studied in the last decade (2,13,16,24,25). Modeling the effects of land-use change on flood has been numerously investigated in the last decades (2). Bahremand (2006) has conducted a research on the effect of afforestation on the flood hydrograph features in one of the watersheds in Slovakia. The results revealed that 51 percent increase in jungles led to 12 percent reduction of Debi peak and also causes nine-hour increase up to the hydrograph peak using WetSpa distribution model (2). Banasik and Fam (2010) have investigated the effects of land-use change on flood hydrograph in a watershed in Netherlands using Curve Number method and Nash model. The results indicated that land-use change in 2007-2009 increased Debi peak from 1.1 cubic meters to 8.92 cubic meters (3). Therefore, regarding the significance and the role of land-use change and also considering the fact that flood hydrograph is the hydrological respond of the watershed to land-use change, in this study, land-use change effects have been studied on flood hydrograph in Jafarabad watershed in Golestan province using GFHM (21) distribution model.
Materials and Methods: In this study, land-use change effects have been studied on flood hydrograph in Ja’farabad watershed using GFHM (21) distribution model. GFHM (21) model is a hydrological temporal and place distribution model. DEM maps, land-use, soil type, and also precipitation data of the watershed are the main data required by this model. In order to calculate surplus precipitation amount, this model applies Kinematic- wave method using curve number method of Natural resources reservation organization (NRCS) and the kinematic-wave method for routing. Based on the conditions of the watershed, four management scenarios were developed with an approach of the effects of destruction and land-use reclamation on flood hydrograph.
Results: The results indicated that 4.2 percent jungle growth (scenario 4), reduces flood Debi peak to 0.55 percent and decreases the flood volume to 2.6 percent. Changing 3.9 percent jungle lands to arid lands (scenario 2) increases flood Debi peak to 0.85 percent and also adds 1.83 percent to flood volume. Also changing 17.8 percent semi-dense lands to water-fed lands (scenario1) can raise flood Debi peak to 7.8 percent and increase flood volume to6.08 percent. The results also depicted that land-use change affects cloudbursts with low Debi, and has more effects on peak Debi and flood volume.
Conclusion: The results revealed that flood Debi and volume can be reduced through non-structural land management in order to lower and manage the floods. Hence, land-use reform plan of watersheds with a jungle reclamation pattern and also changing agricultural land-use into agro-forestry lands especially olive orchards (most similar areas are conducted in Golestan Province) significantly increase early casualties and land capture.