Reducing the hydrological connectivity of gully systems through vegetation restoration: combined field experiment and numerical modelling approach
Authors: Molina A. , Govers G. , Van den Putte A. , Vanacker V. 
Published: April 2010

Abstract. Restoration of degraded land in the southern Ecuadorian Andes has led to alterations in the functioning of degraded catchments. Recovery of vegetation on areas affected by overgrazing, as well as the reforestation or afforestation of gully areas have given rise to modifications of hydrological connectivity within the catchments. Recent research has highlighted the ability of gully channels to trap sediment eroded from steep slopes, especially if vegetation is established along the gully bed. However, vegetation cover not only induces sediment deposition in the gully bed, but may also have a potential to reduce runoff water volume. The performance of gully beds in reducing the transfer of runoff water was investigated by conducting controlled concentrated flow experiments in the field. Experimental field data for 9 gullies were derived by pouring concentrated inflow into the upstream end of the gully channel and measuring the outflow at the downstream end of the channel. Two consecutive flow experiments per gully were carried out, so that data for dry and wet soil conditions were collected. The hydrological response to concentrated flow was estimated for each experiment by calculating its cumulative infiltration coefficient, IC (%). The results showed a great difference in IC between dry and wet soil conditions. The IC for wet soil conditions was on average 24%, whereas it was 60% for dry conditions. Gullies with more than 50% surface vegetation cover exhibit the highest cumulative infiltration coefficients (81% for "dry runs", and 34% for "wet runs"), but runoff transmission losses were not as clearly related to vegetation cover as sediment storage. The experimental field data of 16 experiments were used to calibrate a hydrological model in order to simulate the transfer of concentrated flow along the gully beds. The model is based on (i) the Philip's equation to simulate runoff water infiltration and (ii) the kinematic wave approximation to simulate runoff routing. The model is able to predict the transfer of runoff water generally well, as the error on the predicted total outflow volumes is below 13% for 15 out of 16 cases. The sensitivity analysis indicates that the most sensitivity parameters to predictions of transfer of runoff flow in the gully channel are sorptivity S, hydraulic conductivity K and runoff width W. The high sensitivity of model results to some crucial hydraulic parameters is one of the reasons why the relationships between model parameter values and gully features are relatively weak. The results obtained from the field experiments and the kinematic wave model show that gully systems are key elements in the hydrological connectivity of degraded landscapes. The transfer of overland flow and sediment from the slopes towards the river system highly depends on the presence/absence of vegetation in the gully beds and should therefore be accounted for in assessments of landscape degradation and/or recovery.