Positive and negative human-modified droughts: a quantitative approach illustrated with two Iranian catchments

Abstract. Drought management in the anthropocene is challenging because of the complex interaction between climate variability, land surface processes and human changes in the hydrologic system. In the current research, different kinds of natural and human-affected droughts in the anthropocene, including climate-induced drought, human-induced drought and human-modified drought have been investigated for two Iranian catchments (Kiakola and Eskandari catchments), as an example. We propose a methodology to distinguish and quantify positive and negative human-modified droughts to explore the impact of human interferences on river flow and groundwater. The methodology uses naturalized conditions obtained by simulation modelling as a reference to distinguish the droughts. Positive human-modified droughts happen when human activities alleviate natural droughts, whereas negative human-modified droughts reflect severer drought conditions than naturally occurring. Application of the methodology shows that human activities mostly caused negative human-modified droughts (e.g. decreased river flow in more than 89 % of the events) in the selected Iranian catchments, where the Eskandari catchment had about four times severer human-modified droughts than the Kiakola catchment. Positive human-modified droughts mostly occurred in the wet season, which cannot counterbalance the adverse impacts of the negative human-modified droughts occurring in the following dry season. The proposed methodology enables further quantification of drought in the anthropocene through evaluation of negative and positive effects of human activates on the hydrologic system, which will support water management to reduce drought impacts.