The surface runoff response to land use changes in border watersheds of the Central Serbia

The anthropogenic impact on natural processes changing their intensity and even trends is evident and confirmed in many researches. The continuous population urban-to-rural and rural-to-urban movements bring a series of the other changes in the environment. This research estimates the impact of the changes of land use and land cover in a time interval of the last almost three decades (1990–2018) on the extreme runoff in more than 40 watersheds with a torrential water regime. Extreme rainfall episodes and watershed characteristics (steep slopes combined with the geology and soil cover of lower water infiltration as well as sparse vegetation cover and artificial and agricultural land use) are the triggers of occurrence of the torrential floods as the most frequent natural hazards in the Central Serbia. The focus is on the border regions of the Central Serbia, so the observed watersheds belong to the Drina river basin in the Western Serbia, the Timok and the Danube river basin in the Eastern Serbia and the Južna Morava river basin and the Egej basin in the Southern Serbia. The observed watersheds are selected according to the physical-geographical characteristics as well as their mentioning in the Inventory of the torrential floods in Serbia.

The main hydrological indicator whose changes are examined is the curve number that is used for the assessment of the hydrological response of the ungauged watershed in an event of extreme rainfall episode. The usage of curve number together with the watershed morphometric parameters (including rainfall data) enables the assessment of the maximal discharges in the flood event. The curve number is a core parameter of the Soil Conservation Service (SCS, today Natural Resources Conservation Service – NRCS) method whose value is in the defined range (0<CN<100) depending on land use. The lower CN, the lower runoff and the higher CN, the higher runoff.

The results revealed consequences of population movements, especially emigration in the last decades from the border regions in terms of changing the land use patterns, and consequently changing the curve number of watershed – its decline, CN_IID or growth, CN_IIG. The dominant decline of the curve numbers is recorded in more than 20 watersheds of the border Eastern and Southern Serbia known for its continuous depopulation processes in last several decades. This led to the abandonment of arable lands that turned to the transitional woodland-shrub and forest areas in the course of time which finally results in lowering of the peak discharges in the torrential flood events. The minor changes of the curve numbers of more than 10 watersheds are dominant in the Drina river basin. For the selected watersheds the changes (decrease/increase) of the maximal discharges of 100- and 200-year return period are calculated according to the land use in 1990 and 2018 and rainfall data up to 1990 and 2018. In the torrential flood mitigation, findings related to these spontaneous positive anthropogenic influence on declining the surface runoff should be followed by the implementation of a set of preventive measures in erosion and torrent control.