The impact of contour ridges on runoff from a small catchment
In arid and semiarid Tunisian regions, water and soil conservation is an important way to decrease erosion and to collect runoff in hill-slope catchments. However, few studies have focused on the impacts of runoff on water harvesting techniques. This paper deals with the hydrological effects of contour ridges in the El Gouazine catchment located in central Tunisia. Hydrological observations were used to calibrate a conceptual rainfall-runoff model. The H2U hydrological model was used for this purpose (CUDENNEC, 2000; DUCHESNE et al., 1997). This model is built around a production function that defines the net storm rainfall (portion of rainfall during a storm that reaches a stream channel as direct runoff) from the gross rainfall (observed rainfall in the catchment) and a transfer function based on the most complete possible definition of the surface drainage system (CUDENNEC, 2000). The runoff production function defined by NASRI et al. (2001) was used for the El Gouazine catchment. Observed rainfall during 5-min time steps was used in the model. The 18.1 km2 El Gouazine catchment is located 15 km south of the town Oueslatia. With an annual average rainfall of 411 mm and a potential evapotranspiration of 1680 mm, the climate can be regarded as semiarid. The main annual rainfall is usually received between September and January. All runoff from the catchment is collected in a small artificial lake, created in 1990. Its initial storage capacity was 233,370 m3. Water level observations in the lake were used to estimate total runoff from the catchment. The altitude of the drainage area varied from 575 m in the south to 375 m in the north at its outlet. The land surface of the catchment is partitioned into the following uses: 40% cereal production alternating with fallow land; 33% pasture, shrub-land (carob trees, lentisk), and Esparto grass steppe; 20% Aleppo pine forest (both dense and degraded forest); 6% arboriculture (olive trees, almonds); and 1% lake and dyke area. Between July 1996 and July 1997, soil contour ridges covering 43% of the total area were constructed in the El Gouazine hillside catchment. The length of each ridge is typically about 100 m with an average height of 1.5 m and an average perpendicular distance between ridges corresponding to about 25 m. Construction of the contour ridges significantly changed the hydrological conditions in the catchment. Before contour ridges, average global runoff coefficients were 20-30% for autumn rainfalls (september to october) and 4-10% for rainfalls in winter and spring (November to May). Rainfalls during autumn were usually characterized by high intensity and soils were usually compacted with an impermeable surface crust. From November to February, soils usually become more permeable due to agricultural activities (e.g., ploughing) and during March to May infiltration is increased due to increasing crop cover.During this period, before introducing contours ridges in the El Gouazine catchment, the conceptual rainfall-runoff model based on surface topography and the drainage network was calibrated to reproduce observed runoff satisfactorily. Geomorphologic data (topography and drainage system characteristics) for the catchment were extracted with the geographic information software ERDAS IMAGINE of ESRI (Environmental Systems Research Institute, Inc.). The procedure allows extracting the actual probability density function (p d f) of watercourse lengths in the network (L) for the catchment, the catchment Strahler order (n), and the average length of the watercourse in the network L. These parameters are used in a gamma law type transfer function called H2U (unit hydrograph). For this study, in order to be in the usual situation of flood designing practitioners, we used topographical maps with a scale of 1:50000, which are the commonly available maps in Tunisia, and digital grids with a 10 m resolution. Due to the semiarid landscape and relief, there is no ambiguity in identifying the drainage networks. Simulated runoffs using both the actual and theoretical pdfs were compared. The theoretical functions give better results, especially with respect to peak flow. This is probably due to smoothing of the drainage system represented by the theoretical density function. The empirical function calculated from maps probably includes errors and uncertainties, which are smoothed in the theoretical function. For both calibration and validation events, results showed an excellent agreement between observed and simulated runoff volumes as well as peak flows. For the Nash criteria, results varied from case to case (from 0.62 to 0.96). After introduction of the contour ridges, runoff coefficients changed to below 10%. Similarly, inflow to a reservoir at the outlet of the catchment decreased by 50-80%. Observed erosion virtually disappeared after introduction of the contour ridges. Similarly, peak discharge was reduced 60-90%. The changes made in the semiarid catchment will allow agriculture to more efficiently use existing water resources. In practical terms it means that pasture lands can be diverted into cultivated areas (e.g., cereals, olive trees, and almond trees). However, it also means that most surface water infiltrates and eventually replenishes the groundwater, thus leaving downstream areas with almost no surface runoff. Soil contour ridges covering 43% of the El Gouazine catchment area increased the total runoff storage capacity by approximately 258 000 m3. Consequently, contour ridges may be said to present an alternative to the construction of small hillside reservoirs. The introduction of contour ridges therefore has dramatic consequences for downstream runoff conditions. To improve runoff management for both upstream and downstream conditions, it may be necessary to a) simulate runoff effects using a conceptual rainfall-runoff model and b) allow a certain part of the catchment to deviate a part of the runoff to the downstream natural watercourse. A conceptual rainfall-runoff based on surface topography and the drainage network was seen to reproduce observed runoff accurately. By applying the model for runoff simulation with and without contour ridges a more detailed comparison of the changed runoff conditions could be made. The detailed hydraulic function of water harvesting techniques is still to a major extent unknown. A detailed study of the hydrological function of contour ridges in a cascade system is necessary to calibrate hydrological models such as H2U and their associated production function on hill-slope catchments with soil surface management.