Rooftop rainwater harvesting efficiency modeling based on precipitation climatology of the southern region of Ukraine

Abstract

The southern part of Ukraine is one of the most vulnerable regions of the country to climate change due to the increasing precipitation deficit. The region is highly dependent on a system of canals fed with water from the Kakhovka reservoir. The Kakhovka Dam destruction on June 6, 2023 and the consequent disappearance of the reservoir led to water supply problems to many settlements and agricultural lands. Rainwater harvesting in a precipitation-deficient region is an important measure for climate change adaptation. One of the solutions to mitigate the consequences of technical water deficit at the individual household level is an installation of rooftop rainwater harvesting systems (RRWHS). To evaluate the feasibility of using such systems and estimate the amount of water potentially collected, simulations are performed based on both climatological and climate projections data. Various indicators can be used to evaluate the RRWHS efficiency, e.g. volume of water that can be collected (precipitation inflow PI index) or the amount of time with a shortage of water in the water storage (demand exceeds supply, DES index) for a given roof area, the roof and the water storage geometry and average water consumption. The essence of the DES index is the time in days when water demand exceeds the actual water supply to the RRWHS water storage. The DES calculation involves solving the water balance equation as described in [1]. The water level change component of the equation takes into account runoff coefficient (depends on the roof geometry), the roof area, actual precipitation during a day, water outflow (daily water demand), daily evaporation rate and an area of the water storage [1]. When solving the water balance equation, the condition of not exceeding the actual water storage volume must be met. The E-OBSv27.0e dataset [2] was used as input data on precipitation over the period 1991-2020. The results of DES and PI simulations were obtained for the roof area of 50, 75, 100 m2 under the condition of the runoff coefficient equal to 0.75. The results were obtained for a fixed daily water demand of 0.3 m3 and the water storage volume of 2.0 m3. For climatological assessment, the daily DES data in the southern region of Ukraine were averaged over months, seasons and the whole climatological period. Since the efficiency of RRWHS depends on the roof area, for practical purposes PI index per 1 m2 of the collection area was calculated as well. The simulation code has been adapted to use both GFS and WRF forecast data as an input. Thus, it can be used for operational forecasting of the water level in the water storage of preset parameters in different sites of the Southern region of Ukraine.