Water quality in small agricultural slopy catchments in Bohemian - Moravian highlands is often threatened by high nitrate concentration and nitrogen loads from diffusive agricultural sources. The main goal of this thesis was to get a practical evidence for findings obtained in the Czech Republic and abroad by statistical approaches concerning the profoundly mitigative effects that grassing certain catchment areas has on the nitrate burden in drainage and surface waters. The research was conducted mainly in the Dehtáře experimental catchment, where part of the recharge area with an area of 4.6 ha, has been grassed since the hydrological year 2007. Together with long-term monitoring of nitrate leaching in subcatchments with different land use in recharge and discharge are, drainage runoff generation and composition were surveyed in order to find knowledge about the recharge area placement and its importance for drainage water quality. The possibility of using the continuous measurement of water temperature for drainage runoff separation was also tested. The results presented show that nitrate concentration values in drainage water were influenced the most by the land use of the recharge zones within the drainage subcatchment. While permanent grassland placed directly in the drained area (corresponding to the catchment discharge zone) did not show any influence, the grassing focused on the catchment recharge area demonstrated a significant decrease in both, NO3 concentrations and N loads. Approximately one year after grassing, the long-term course of nitrates concentrations changed direction and became decreasing in sites with completely or partly grassed recharge zone, while in sites without land use change), the nitrate concentrations trend remains increasing or the stagnation was found. Decreases by 32% was detected in systematic drainage subsystem, which recharge area was completely grassed. The nitrate-nitrogen leaching decreased after grassing in recharge area of the drainage system by 23%. These findings can be generalised for slopy agricultural catchments with common land use in soil environments formed on crystalline rocks. The acquired findings are of high importance for improving the water quality of small streams as well as groundwater in agriculturally exploited areas, for planning protective zones within large catchments of potable water reservoirs, and also for protecting small local surface or groundwater sources of potable water.