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The purpose of this project was to study available methods for runoff calculation for determination of discharge to ditches in smaller agricultural watersheds. The aim was to determine future drainage design through ditches in agricultural catchments < 500 ha. 

The SCS Curve Number Method is a widely-used method for predicting direct runoff from single storm events by using curve numbers (CN). CN describes the runoff potential as a function of land use and cover type and hydrologic soil groups (HSG). The curve number method was applied on two agricultural watersheds, 1544 and 845 ha respectively, in Sweden. This was done in order to determine  how well variations in direct runoff potential could be described as well as direct runoff events could be predicted by using the method. A GIS-based analysis of cartographic materials was performed to create a classified raster for land use and soil groups for each watershed. Soil groups in the map layer from SGU were classified into HSG, mainly by a comparison of the theoretical permeability for HSG and Swedish soil groups. Using an overlay analysis in GIS, raster’s where combined for determination of appropriate CN. Daily direct runoff were calculated from daily precipitation with alternating CN depending on the 5 days’ antecedent precipitation. To be able to estimate peak flows from direct runoff the method was combined with a modified version of the rational formula used in the SWAT-component for surface runoff. The time of concentration was estimated by modelling travel time parameters in ArcSWAT from a digital elevation model with 2 meter resolution. Fractions for precipitation describing the rainfall intensity 𝑎0,5, were generated from a stochastic triangular distribution and series of daily precipitation. 

Variations in direct runoff potential could be seen for sub areas less than 100 ha. Such variations were mainly assumed to be a result of variations hydrologic soil groups. As for the whole catchment, variations in direct runoff potential was believed to be smoothed out since the catchments were comparatively large and had similar land use. Using rainfall amounts and a definition for the antecedent soil moisture conditions runoff events were well estimated in both areas for periods when precipitation fell as rain and the soil was not frozen. An adjustment of CN during periods with soil frost was discussed to improve the method. Soil moisture levels may also be investigated for periods with soil frost thawing and low evapotranspiration. In terms of runoff amounts and available data, validation of the method on the studied areas was difficult. The modified rational formula both under- and overestimated measured flow rate which is explained by the fact that the curve number method does not include all flow paths in the agricultural landscape, but also by uncertainties in the rainfall intensity parameter 𝑎0,5,. Possibilities for calibration of the modified rational method must be investigated before the method can be recommended for peak flow estimation. For the time being, graphical estimations of discharge from natural areas according to Svenskt Vatten’s publication P110 (2016) is recommended for evaluation of future ditch drainage design.