Investigation of Land use Changes and Its Role in the Hydrology of the Upstream Areas of Siahroud Watershed

Land use changes are one of the important factors in changing the hydrological status of the watershed and soil erosion, so the type of land use is one of the important and determining factors in the production of surface runoff. By knowing the process of land use changes, it is possible to manage the effects of land use changes in the hydrology of the affected areas and led the natural ecosystems towards balance. The following research was carried out with the aim of revealing land use changes using satellite data and analyzing its role in changing the hydrological situation.

The study area is the upstream part of the Siahroud river basin located in the south of Qaemshahr city. In this research, to extract land use maps of different time periods, integrated pixel-based and object-oriented methods, unsupervised and supervised classification of TM sensor images of Landsat 5 satellite related to 1998 and 2009 and OLI sensor images of Landsat 8 satellite related to 2015 and 2019 were used. After preparing the land use maps for each time period, the CN values for each land use were determined based on the standard instructions of the SCS method and the weighted average CN was estimated for the entire study area. At the end, the amounts of runoff production and depth of runoff resulting from rainfall are calculated.

The land use changes of the studied area in the period of 22 years showed that 281%, equivalent to 657 hectares, was added to the area of citrus orchards. About 5% of the area of deciduous forest has decreased and the area of residential areas has increased by 57%. Due to the changes in land use during this 22-year period, the weighted average value of CN has decreased from 63.01 to 62.73. Also, the estimation of the amount of runoff for rainfall equal to 59.8 mm (average maximum rainfall of 24 hours during the period of 22 years) showed that the depth of runoff has decreased from 4.83 mm in the 1988 situation to 4.73 mm in the condition of 2019.

Considering the changes in land use and its effect on watershed runoff production, it seems that these effects have been moderated and land use changes have played a role in reducing runoff production over a period of 22 years. Therefore, it is expected that with the continuation of this process of land use changes, the production of runoff in the upstream of the Siahroud River watershed will decrease.