Spatial development of hydrological droughts in lake catchments

In river-lake systems, reservoirs significantly affect the formation of water resources in the catchment by reducing the maximum flows and floods as well as increasing the low flows. In lake catchments, the pace of hydrological drought progression is usually slow. However, this phenomenon can be very extreme and destructive for water balance structure as a result of the very slow renewal rate of catchment resources. An estimation of hydrological drought development was conducted in the Biebrza river catchment (6,900 km2) on the basis of a daily discharge series for 18 water-gauge stations in the period 1982–2014. The number and location of the gauges allowed reliable results of the spatial pattern of drought to be obtained. The main objective of the research was the construction and assessment of the applicability of indicators which are estimators of drought spatial progression. Comparative analysis of four proposed indices led to two of them being recommended. These characteristics estimate the direction of drought development according to the stream network hierarchy, location of sub-catchments and the direction of river basin area increase. As a result, determinants of the spatial development of hydrological drought as well as its importance in the hydrographical structure were identified. Furthermore, the intensity of drought was evaluated and all investigated indices were applied to a time series analysis