Test and further develop a user-friendly Watershed Tool

The WOCAT Sustainable Land Management Watershed Tool (WT) allows to assess and illustrate the impacts of different land management practices on the “runoff” of small to medium-sized watersheds following a precipitation event. It is based on the Soil Conservation Service Curve Number methodology (SCS-CN). The method has only one parameter – the Curve Number (CN) – and is designed to be applied in ungauged watersheds. Based on several watershed characteristics (hydrologic soil group, land use and cover, hydrologic condition, structural measures, and slope) the representative CN can be estimated. Furthermore, it is possible to account for antecedent moisture conditions (AMC). AMC accounting is a three-staged process. AMC I represents dry, AMC II average, and AMC III wet conditions. The initial CN estimate is taken to be representative for AMC II. Suitable CNs for AMC I and AMC III are calculated based on the initial estimate. The SCS-CN method can be applied in different forms. In the composite form a weighted CN is calculated for the whole watershed. In the distributed form, the “runoff” is calculated for smaller cells constituting the watershed. In this thesis the opportunities and risks of the application of the WT are investigated and issues of its application are highlighted and discussed. A validity assessment of the WT is conducted through three case studies, whereby the SCS-CN method is applied in different forms – the composited, the distributed, and the calibrated form. Throughout the thesis, a special emphasis is placed on the application of the WT in different climatic settings and biomes. The first case study analyses three small, forested, very humid, and steep watersheds in the Orinoco river basin in Columbia. The second case study examines three small watersheds in the Ewaso Ng’iro close to the Mount Kenya, differing in terms of yearly precipitation, land cover and use, and slope. In the third case study the SCS-CN method is applied to 11 Swiss watersheds of different sizes. Across all three studies the SCS-CN generally provided acceptable “runoff” estimates with average mean absolute errors of around 10mm for events above 50mm in the Switzerland and Columbia case study. Through the case studies the following hypotheses about the SCS-CN method are investigated: 1. There is little to no relationship between the estimated CNs and the calibrated CNs for large sets of watersheds. (Hawkins, 1984) 2. The composited CN approach is generally less accurate than the distributed approach as it underestimates the representative CN. (Grove et al., 1998) 3. For each observed precipitation-“runoff” pair an ideal CN can be found. Event-wise ideal CNs tend to follow trends with changing precipitation, often decreasing with increasing precipitation until a near-constant value is approximated. This is called secondary P effect. (Hawkins, 1993) 4. It is claimed that AMC I and AMC III act as a surrogate for all other sources of variability. They approximate the bounds at the 90% and 10% exceedance frequency of event-wise ideal CNs in the near-constant CN range, respectively. (Hjelmfelt et al., 1981) The case studies showed the following: 1. Not confirmed for all examined watersheds (with the exception for one Swiss watershed) the representative CNs were correctly estimated. On average, the CN estimates differed by about 4. 2. Not confirmed although the composited CN approach always leads to a lower “runoff” estimate, it is not possible to conclusively state that one approach is better. In the Swiss case study, the distributed approach provided more accurate results for 4 of 11 watersheds. In the Columbian case study the composited CN performed better for all three watersheds. 3. Confirmed – in every analysed watershed a secondary P effect was identified . This emphasises that the WT should only be applied to large events. 4. Tentatively confirmed – in line with the hypothesis, in the Swiss case study, the CNs for AMC I and AMC III on average approximated the 88% and the 9% exceedance frequency. In the Columbian case study a similar tendency could be identified, however for a full assessment additional data is required.