Differences in stream flow in relation to changes in land cover: a comparative study in two sub-Mediterranean mountain catchments.

Abstract

The stream flow response of two neighboring catchments in the central Spanish Pyrenees was compared for 26 rainstorms covering both catchments: one catchment (2.84 km2) was extensively used for agriculture in the past, and the other (0.92 km2) is covered by dense natural forest. Their similarity in terms of lithology and topography enabled us to separate the effects of soil and land cover on their hydrological responses. Relative to the forested catchment, peak flows in the past agricultural catchment were always greater (566 vs. 119 l s−1 km−2), the response time was 2- to 3-fold faster (131 vs. 356 min), and the recession limbs were 1–2 orders of magnitude shorter (7 vs. 72 h). Storm flow was usually greater in the former agricultural catchment, especially for low–intermediate sized flood events; only for larger events the storm flow in the forested catchment was sometimes greater. Storm flow differences were closely related to catchment wetness conditions and showed a marked seasonal pattern, with higher values in the past agricultural catchment under dry conditions, and usually higher values in the forested catchment under wet conditions. In the past agricultural catchment, runoff was generated during the entire water year, through both surface (i.e. infiltration excess and saturation excess overland flow) and subsurface flow. We suggest that the forested catchment can be characterized by a dual (or switching) behavior controlled by soil moisture conditions, which regulates the hydrological connectivity and favors the release of large amounts of subsurface flow. Differences in soil depth and permeability, together with differences in vegetation cover, may explain the contrasting dominant runoff generation processes operating in each catchment, and consequently the differences between their hydrograph characteristics.