From meteorological to reservoir drought: The influence of a dense network of unmonitored small reservoirs on drought evolution in a semiarid region

Abstract:

The construction of reservoirs without adequate basin-level planning can cause a concentration of water in a certain part and scarcity somewhere else. This can give rise to internal conflicts, and societal pressure to build more reservoirs, creating a vicious cycle. Whereas larger reservoirs are generally planned and managed, especially small, unmonitored and private reservoirs can play a role in this vicious cycle. In this study, we analyzed an unmonitored dense network of small reservoirs in a semiarid north-East Brazil catchment, and evaluate the role of this in the evolution of a drought.

First, we propose a method to identify and quantify water storage in a network of unmonitored small reservoirs in the Riacho do Sangue Watershed. We employed Landsat 5, 7, and 8 images to identify and analyze the water extent of small reservoirs in this basin, and then we used this as input for an elevation area capacity curve defined using the Tandem-X DEM (10m). We estimated that there are almost 2900 small reservoirs in the study area (2 reservoirs/km²) and that they represent over 50% of total storage capacity in monitored reservoirs in this region.

Secondly, we evaluated the role of these small reservoirs in the distribution of water during and following a period of meteorological drought. For this, we combine Standardized Precipitation Index with Volume Deficit information (deviation from the half of total capacity of the monitored reservoirs). We noticed that during intense hydrological drought (e.g. 2012-2017 event), the small reservoirs slightly impact the large ones since they were almost empty.. However, we could track the delay in the recovery of the volume stored in the large reservoir due to the presence of the small ones. This delay causes an artificial persistence of the hydrological drought in this large reservoir, even when precipitation is back to normal. We termed this a reservoir-network drought and concluded that for the Riacho do Sangue Watershed, small reservoirs can prolong the impacts of drought in the large reservoirs for more than 6 months.

This study demonstrates the dynamic between small reservoirs, often constructed in response to drought, and the subsequent impact on large reservoirs. This phenomenon in highly relevant for the functioning of formal water management regulations developed for large reservoirs.