Using NDVI as an indicator of snowmelt runoff in Mediterranean mountain catchments: application to the Guadalfeo river basin, Sierra Nevada.

In Mediterranean high mountain environments, the snow accumulation/melting regime determines the surface water balance, redistributing the fractions of infiltration/runoff and evaporation from the ground and conditioning the volume circulating in the river channels. Therefore, the snow cover adds another degree of complexity to the precipitation-flow relationships in these systems (e.g., rain-on-snow events). Thus, a process-oriented approach is necessary when studying these relationships.

The definition of indicators derived from satellite remote information is a widespread technique in the characterization of the hydrological state of a basin. Previous studies in these areas have identified the state of vegetation, through its vigorousness using the NDVI (Normalized Difference Vegetation Index), as a good indicator for the characterization of soil water content at the beginning of the hydrological year (Gómez-Giráldez et al., 2014).

Therefore, in this context, the objective of this study is to analyze the relationships between changes in the state of watershed vegetation and flow variations in snow-influenced watersheds as additional information to anticipate downstream seasonal inputs using previous remote sensing information. For this purpose, a 20-year series of daily flow rates at a gauged point in the Guadalfeo river basin, one of the headwater catchments of the Sierra Nevada National and Natural Paro river was used, together with meteorological time series and the simulation of snowpack water flows using the physical-based model SNOWMED. Hence, two flow events are selected: i) rainfall-runoff events and ii) melt-runoff events. The vegetation dynamics in the basin is analyzed through variations of NDVI, derived from Landsat data (TM and ETM+) during the same study period.

From the analysis, the lags between the local change in NDVI, the loss of snow-covered area at the headwaters and the occurrence of melt or mixed flow events allow us to establish the onset of the generalized melt state in the basin, as well as to estimate threshold antecedent states. The results deepen the understanding of the hydrological dynamics in the basin, as well as the capacity of the NDVI as a tracer of the hydrographs associated with the generalized states of melting in Mediterranean mountain basins.