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Potential evapotranspiration (ETP) is an important part of a climatic water balance and a crucial variable in many kinds of models in computing actual evapotranspiration. The objective of this study was to find a reasonable approach of ETP calculation for a height-differentiated landscape in subtropical climate. From the pool of diverse approaches, six common methods [Hamon, Priestley-Taylor, Thornthwaite, Blaney-Criddle, Turc and Food and Agricultural Organization Irrigation and Drainage Paper No. 56 (FAO-56)] were selected. With the meteorological data for long-term period , the calculation of ETP values was performed for 12 different meteorological stations in the Red River basin. Among the applied ETP calculation methods, the Turc and the FAO-56 methods agreed well at most stations and represent best the expectations for the ETP values of the Thao and Da subbasins. The findings of our investigations indicate that in highly structured (land use and elevation) regions, not all methods provide satisfying results.

Abstract:The objective of the study was to evaluate the spatial distribution of peakflow pre-event water contributions and streamwater residence times with emphasis on land use patterns in 38 subcatchments within the 687 km 2 large mesoscale transboundary catchment Lužická Nisa. Mean residence times between 8 and 27 months and portions of preevent water between 10 and 97% on a storm event peakflow were determined, using 18 O data in precipitation and streamwater from a weekly monitoring of nearly two years. Only a small tracer variation buffering effect of the lowland tributaries on the main stem was observed, indicating the dominant impact on the mountainous headwaters on the runoff generation. Longest mean streamwater residence times of 27 months were identified in the nearly natural headwaters of the Jizera Mountains, revealing no ambiguous correlation between the catchment area and altitude and the mean residence time of streamwater. Land use control on the pre-event water portions were determined for three land use categories with percentage of urban areas from 0 to 10%, 10 to 20% and more than 20%. The fraction of pre-event water in the first category decreases from 97% to 65% with the increasing percentage of forest from 76% to 100%, revealing that forests may provide only a limited infiltration of precipitation due to leaf interception and soil water use for transpiration. Fractions of pre-event water of 39-87% in the second (agricultural catchments) and of 10-35% in the third (urbanized catchments) category increase with percentage of non-urban areas.