Investigation of the Effect of to Land Use Changes on Flood Hydrograph and Fluctuations of Groundwater Level in Part of Ghareh Souh Basin

One of the factors affecting the surface runoff characteristics and the fluctuations of groundwater in the watersheds is the land use change at different levels of the basins. Land use change can also be a conversion from one user to another, or, in other words, changes in the composition and pattern of land use in a region, or the modification of a particular type, is land use change (Jakeman et al., 2005). Incorrect land use changes will disrupt the water cycle from natural equilibrium, which will result in damage caused by devastating floods, including economic losses, loss of life, waste of water, and consequently the loss of water resources. Land use changes also affect surface and subsurface flow changes (Palamuleni et al., 2011). On the other hand, groundwater, like other natural resources (both quantitatively and qualitatively) is subjected to the threat of human activities. Underground water drainage and depletion of groundwater aquifer and its consequences including increased water harvesting costs, land subsidence, and water quality has declined across regions of the world. In recent years, several studies have been done on land use change. For example Zou et al. (2016) detected hydrological changes using statistical tests, SWAT, and land use maps. The results show significant decreasing trends in both annual runoff and sediment loads, whereas slightly decreasing and significantly increasing trends are detected for annual precipitation and air temperature, respectively. Akter et al. (2018) investigated the hydrological behavior due to the urbanization under current and future climate scenarios of high summer and high winter rainfall for 20 sub-catchments of the Schijn River, in Belgium. Results revealed that peak flow for urban runoff and the total peak flow (i.e. rural and urban runoff) were significantly higher (i.e. ranges from 200% to 500%) than the existing rainfall-runoff model (PDM) flows because of faster and more peaked urban runoff responses. Lee and Brody (2018) examined the impact of land use on flood losses in Seoul, Korea. Results indicate that urban built-up land with higher impervious surfaces and agricultural land may cause more flood damage than other land uses analyzed in the study. However, a high density development of compact design can decrease flood loss. These results indicate the importance of resilient land use planning in urban areas. Jodar-Abellan et al., (2019) investigated the impact of land use changes on flash flood prediction using a sub-daily SWAT model in five Mediterranean ungauged watersheds. The results showed that with increasing flow rate peaks and decreasing concentration time and high urban development along the Spanish coastline in the studied basin, the curve number increased and the discharge rate exceed 900 m3/s. The aim of the current research is to study the effect of different land uses and its changes during the years 1371, 1381, and 1391 on surface run off, flood, and groundwater fluctuations in a part of Ghareh Su, Ardabil watershed.

The study area in this study is part of Ghareh Souh basin in Ardebil province with an area of 2162.62 square kilometers in the geographical area of 48º 27ʹ 36" and 49º 12ʹ 06" east longitude 37º 47ʹ 20" and 38º 37ʹ 25" in the north latitude. The minimum and maximum altitudes of the watershed are 1280 and 3829 meters above sea level respectively, and the average slope is 11.57 percent. Land use in this area often includes forest, water farming, wasteland land, rangeland, dry farming, and urban land uses. Among the types of land uses in the studied basin, the use of dry farming has the highest level of the basin. In this research, satellite images needed for the years studied (1371, 1381 and 1391) were prepared from the site of the US Geological survey. After providing satellite imagery for the study area, ENVI software was used during different stages of image processing. For this purpose, maximum likelihood classifier was used to study vegetation changes. Before analyzing satellite data, it is necessary to make corrections on raw images. In remote sensing, raw data errors are sensor errors and atmospheric effects. To correct the error, the method of atmospheric correction and systematic geometric correction are used. At first, the maps of land use and curve number in the mentioned years was gathered and the area of each of units was extracted. In continue, the process of land use changes in the cases of study period and its effect on changing the curve number and height of runoff and then stimulation of flood by the model of HEC-HMS and drawing of the hydrograph was done. In the end, the effect of land use changes on groundwater level was studied. To calculate runoff from annual precipitation, the average annual precipitation of 14 rain-gauge stations was used in the part of the Ghareh Souz basin.

In this study, permeability was obtained using a soil map of the region that included soil texture characteristics. Then, due to soil permeability, a map of soil hydrological groups (A, B, C and D) was extracted. It is observed that the hydrological group C has the largest area, which is equal to 49.82% of the basin, and the hydrological group B also includes 72.74% of the basin area. Therefore, due to the fact that the hydrological group C has the largest area in the basin area, the studied basin is more capable of runoff production. Regarding the results, it can be seen that the studied area is more moderate and relatively low permeability classes in terms of permeability. The results showed that total losses in 1371 was 69.28 mm, which in 1381 was 66.95 mm and in 1391 it decreased to 64.42 mm. The amount of runoff was reduced which was 263.4 mm in 1371, but in the year 1381 the amount of runoff increased to 294.785 mm which in 1391 it increased to 297.07 mm . The increase in runoff in the studied basin can be due to an increase in the number of curves. The results showed that during the case of study period, forest areas, water farming, and wasteland uses were decreased to 2.54%, 16.69%, and 1.19%, respectively and the area of the rangeland, dry farming, and urban land uses increased to 5.74%, 12.39%, 2.29%, respectively. This changes have caused the increase of curve number from 78.57 to 79.77 in the year 1371 and 1391, respectively and following the decrease of total losses from 69.28 millimeters in the year 1371 to 64.42 millimeters in the year 1391. Also, run off height has increased from 263.4 millimeters in the year 1371 to 297.07mm in the year 1391 (11.33%). Considering the curve number and flood stimulation, the achieved results showed that peak discharge and flood volume have increased to 9.49% and 6.67%, respectively. In the end, these changes have led to a decrease of groundwater level to 6.14 meters (39.06%) during the studied period.

Due to the increased utilization of urban land as well as the increasing dry farming and underground water withdrawals, by decreasing the permeability of the basin soil, the amount of flood and runoff has increased and eventually we face a drop in groundwater. Calculation of correlation coefficient between different land uses and curve number, runoff height, flood volume, and groundwater level showed that these variables have direct relation with rangeland, dry farming, and urban land uses while they have inverse relationship with forest, water farming and wasteland.