1. Site description

The Nučice experimental catchment was established in 2011 with the main aim to study water balance of the cultivated fields and rainfall-runoff and soil erosion processes. The catchment is 0.531 km2and located 30 km east of Prague in an intensively agricultural landscape in the Central Bohemian Region, Czech Republic (catchment outlet location 49°57’49.230” N, 14°52’13.242” E). The morphological management and climatic conditions are representative for farmlands of the Czech Republic. The area belongs to the moderately hilly Bohemian Massif, the catchment has the average altitude of 401 m a.s.l. and slopes ranging from 1 % to 12 %. The climate is humid continental with average annual precipitation of 630 mm from 1975 to 2015, potential evapotranspiration between 500 and 550 mm, and mean annual air temperature of 7.9 °C . The catchment is drained by a 950 m long, narrow stream which has been piped in the uppermost part. The channel has a trapezoidal cross-section that is 0.6 m wide at the stream bed with an average depth of 1.5 m (Figure 1).
The area of the catchment is almost exclusively covered by arable land. Less than 5 % of the area consists of the stream, paved roads and shrublands. The fields are tilled to the edge of the stream banks; grass strips are missing, therefore the surface runoff and eroded soil may enter the stream without significant transformation in the riparian zone. The catchment is divided into three parcels which have existed since 2000. The standard crop rotation is dominated by winter wheat (Triticum aestivum L .), rapeseed (Brassica napus ), summer oats (Avena sativa ) and alfalfa (Medicago sativa ).
The soils are developed on Paleozoic conglomerate with arenaceous and rudaceous texture and are classified as Haplic Luvisols and Cambisols. The soil texture is considered sandy loam (9% clay, 58% silt, and 33% sand on average). Several geophysical surveys using electrical resistivity tomography (ERT) have been conducted to capture the degree of homogeneity/heterogeneity present in the compacted layer and to determine the depth of the bedrock . Based on geophysical monitoring and a geological borehole survey conducted at a close-by location, the bedrock ranges in depth from 6–20 m. The soil is tilled to approximately 12 cm, below the tilled topsoil there is a compacted plough pan with very low hydraulic permeability.
The catchment is often very dry during the summer and the baseflow recorded at the closing profile declines to 0 – 0.2 l s-1, in winter and early spring the baseflow is around 4 l s-1. The average annual runoff coefficient is 1 %. The runoff formation during the rainfall events exhibits threshold behavior. Based on the measured rainfall-runoff data, we have identified rather scattered rainfall-runoff relationship with a strong dependence of the runoff on the actual topsoil saturation. Different runoff pathways and runoff mechanisms have been observed. Once the soil moisture conditions are below a certain threshold value, the magnitude of the stormflow is not correlated to rainfall totals. Therefore, the shallow topsoil and its water holding capacity play a significant role in runoff formation . As the topsoil becomes saturated on a large part of the catchment, water is quickly routed via surface (especially through the compacted wheel tracks in the slope wise direction) and shallow subsurface runoff toward the drainage channel. Even though the channel is straight and short, the hydrographs and especially sedigraphs are transformed strongly during the summer events due to dense vegetation on the stream banks .