Drought-generating processes

Hydrological processes are described that lead to anomalies (drought) in river flow and groundwater. We start with an elaboration of the catchment water balance for the different systems (soil, groundwater and surface water) in terms of anomalies. The chapter then describes the hydrological soil processes (infiltration, actual evapotranspiration and groundwater recharge) that control the propagation of a meteorological water balance deficit (due to low rainfall and high potential evapotranspiration) for various soil characteristics, land use and water-table conditions. In cold climates, soil frost, snow accumulation and melt affect soil infiltration and overland flow. Propagation of below-normal recharge into groundwater anomalies (groundwater discharge, levels and spring flow) is discussed for quickly responding (local flow systems) and slowly responding groundwater systems (regional flow systems). Next, we explain how anomalies in overland flow and groundwater discharge may affect anomalies in river flow. The role of lakes in sustaining river flow during low flow is described. The response in hydrological processes during drought in different components of the hydrological system is illustrated with data from two catchments with contrasting climates. We conclude with the introduction of a drought typology that classifies causing factors and propagation processes.