Rainfall-runoff events significantly influence water runoff and the loss of pollutants from tile-drained agricultural land. We monitored ten small (4 to 38 ha) tile-drained catchments in Czechia for three to five years (2012 to 2016). The discharge was measured continuously; a regular 14-day scheme of water quality monitoring was accompanied with event sampling provided by automatic samplers in 20 to 120 min intervals. A new semi-automated algorithm was developed for the identification of runoff events (RE) based on discharge and water temperature changes. We then quantified the share of RE on the total runoff and the N and P losses, and we compared six methods for nutrient load estimation on an annual and monthly basis. The results showed considerable differences among the monitored sites, seasons, and applied methods. The share of RE on N loads was on average 5% to 30% of the total annual load, whereas for P (dissolved and total), the share of RE was on average 10% to 80% on the total annual load. The most precise method for nutrient load estimation included the RE. The methods based on point monitoring of the discharge and water quality underestimated the loads of N by 10% to 20% and of P by 30% to 80%. The acquired findings are crucial for the improvement of nutrient load assessment in tile-drained catchments, as well as for the design of various mitigation measures on tile-drained agricultural land.

Abstract:Tile drainage water temperatures and discharge rates were measured in five highland watersheds of which most are underlain by acid crystalline rock. One of them, Dehtáře in the Bohemo-Moravian highland (Czech Republic), was studied in greater detail. The aim was to evaluate water temperature monitoring as a means of determining the source and pathway of drainage runoff during high-flow events. Rapid increase in drainage discharge was accompanied by rapid change in water temperature. In winter, the rising limb of the hydrograph was accompanied by a decrease in temperature, and the falling limb was associated with a corresponding temperature increase. In summer, the trends were reversed. These data suggest that the water temperature changes are caused by the fastest component of drainage runoff, water from a precipitation event or snowmelt, which can be separated from the remainder of the hydrograph. Measurements of hydraulic conductivity, soil moisture content, soil temperature, and groundwater table level indicate that the major portion of the event water causing this effect infiltrates in the watershed recharge zone where soils are permeable, enters the weathered bedrock, flows preferentially and rapidly down the slope along disjoint fissures in the bedrock, finally emerging as ascending springs, and is, for the most part, intercepted by the tile drainage systems.

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Physical properties of soils have a significant influence on their water regime and should be considered when selecting suitable agricultural crops for particular sites, taking into account the crop productivity and its water requirements. Mean daily rates of actual evapotranspiration (ETa) were obtained by collation of measured or otherwise estimated 10-min values for the years 2004, 2006 and 2009 for a partially tile-drained agricultural experimental catchment in the Bohemo-Moravian Highland (Czech Republic). ETa was measured using the Bowen ratio (β) and energy balance (BREB) method at four weather stations located on different soil types (Stagnosols, Cambisols) and terrain relief positions (defined with respect to the groundwater recharge and discharge zones) over different crops (cereals, oil rape and permanent grassland). A systematic influence of soil properties on the evapotranspiration rate was more pronounced during the periods of limited transpiration (soil drought, crop maturity), when the average daily ETa was significantly lower and the corresponding β significantly higher over coarser-textured soils (shallow Haplic Cambisols), namely When the transpiration was not limited by soil water deficit, it acted as an equalizing factor smoothing down evapotranspiration from heterogeneous soil areas, whereby the effect of the soil physical properties was masked. With regard to soil water regime and evapotranspiration, the tile-drained Stagnic Cambisol lands behaved similarly as non-drained Haplic Cambisols. The effects of land use and of the terrain relief position could not be tracked independently, because the permanent grassland was situated in the wetter and texturally heavier parts of the catchment and the positions of the recharge/discharge zone coincided with the positions of texturally lighter/heavier soils, respectively.

Dynamics of pesticides and their metabolites in drainage waters during baseflow periods and rainfall-runoff events (RREs) were studied from 2014 to 2016 at three small, tile-drained agricultural catchments in Bohemian-Moravian Highlands, Czech Republic. Drainage systems in this region are typically built in slopes with considerable proportion of drainage runoff originating outside the drained area itself. Continuous monitoring was performed by automated samplers, and the event hydrograph was separated using O andH isotopes and drainage water temperature. Results showed that drainage systems represent a significant source for pesticides leaching from agricultural land. Leaching of pesticide metabolites was mainly associated with baseflow and shallow interflow. Water from causal precipitation diluted their concentrations. The prerequisites for the leaching of parental compounds were a rainfall-runoff event occurring shortly after spraying, and the presence of event water in the runoff. When such situations happened consequently, pesticides concentrations in drainage water were high and the pesticide load reached several grams in a few hours. Presented results introduce new insights into the processes of pesticides movement in small, tile-drained catchments and emphasizes the need to incorporate drainage hydrology and flow-triggered sampling into monitoring programmes in larger catchments as well as in environment-conservation policy.