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 .