The effect of land-use changes on the hydrological behaviour of Histic Andosols in south Ecuador

The south Ecuadorian Andean mountain belt between 3500 and 4500 in altitude is covered by a highly endemic and fragile ecosystem called paramo. The Histic Andosols covering this region have highly developed hydric properties and exert a key function in the hydrological regulation of the paramo ecosystem. Unlike most Andosols, their extreme water retention capacity is not due to the presence of typical minerals such as allophane or imogolite. Although these minerals are virtually absent, the large organic carbon content, due to organometallic complexation, gives rise to similar proper-ties. The water content at 1500 kPa can exceed 2000 g kg(-1), and the high hydraulic conductivity at saturation (about 15 nun h(-1)) drops sharply when low suction is applied. The three methods applied, i.e. the inverted auger hole, the tension infiltrometer and the constant-head permeameter method, give very similar results. The paramo is characterized by a slow hydrological response and a good water regulation, caused by the combination of a high water storage capacity and high conductivity. The wide pore size distribution of the organometallic complexes results in a water retention curve that differs significantly from the classic Mualem-Van Genuchten description, but can better be described with a simple linear or semilogarithmic model. The soils investigated are very prone to irreversible structural changes caused by land-use changes. The conversion of natural land for cultivation has a large impact on the hydrological function of the region. The water storage capacity increases by 5 to 30%, and the hydraulic conductivity is 31% higher in cultivated catchments. These changes are related to a larger peak flow, a smaller base flow and aenerally a smaller discharge buffering capacity, despite the higher storage capacity. Copyright (c) 2005 John Wiley & Sons, Ltd.