Assessment of Groundwater Drought in the Mangyeong River Basin, Korea

Abstract

When groundwater drought occurs, baseflow discharges to surface-water bodies will be reduced and then domestic and agricultural water usage becomes at risk of insufficient supply. Thus, in this study, several methods for groundwater drought assessment were tested with long-term monitoring water-level data in the study area to preserve groundwater sustainability from drought, principally caused by reduced precipitation and propagated through agricultural drought and groundwater drought. Because of the Monsoon climate on the Korean Peninsula, the groundwater storage (or water-level) is secured until the end of summer, then falls by natural discharge during the dry seasons of autumn, winter and the following spring. Thus, the rainfall in the wet season seems to mainly influence groundwater storage until the spring of the following year. As the groundwater level (GWL) declines due to natural drainage and the use of agricultural water increases by the end of the dry season (October-May), the GWL will become lowered below the critical level. Below this level, sufficient water supply is not secured. Using the Standardized Precipitation Index (SPI), threshold method and 95% probability occurrence method, drought detection and the frequency of drought are compared. Groundwater drought using the threshold method results in more frequent occurrence than using the SPI method. The 95% occurrence method responds to severe drought but it also has weakness in missing the man-induced GWL decline in every spring season. For groundwater drought assessment, an appropriate drought index should be utilized according to climatic conditions and catchment characteristics. In the study area, variations of the both natural and anthropogenic effects are mixed and the threshold method is more suitable as a measure for preventing water resources shortage.

Asymmetric impact of groundwater use on groundwater droughts

“…This constant delay is included in the calculated SPI Q -SGI correlation by the optimal precipitation accumulation period that represents a long-term relationship for a certain site, as both the SPI and SGI were calculated for a continuous 30 year period, including all seasons and both anomalously dry and wet periods. The SPI Q -SGI correlation can be reduced either when groundwater level response becomes disconnected from driving precipitation under confined conditions (Bloomfield et al, 2015;Kumar et al, 2016;Lee et al, 2018) or when groundwater abstraction changes groundwater storage and levels independent from driving precipitation (Bloomfield et al, 2015;Lorenzo-Lacruz et al, 2017;Haas and Birk, 2017). In this study, the impact of confined conditions on reducing SPI Q -SGI correlations is expected to be minimal, as only a small selection of the chalk sites are located in the semi-confined chalk in south Lincolnshire (Table 1).…”

Asymmetric impact of groundwater use on groundwater droughts

“…Under near-natural conditions, the maximum correlation between standardised precipitation and groundwater indices (SPI Q -SGI) is generally high, given an optimal precipitation accumulation period (Q) that accounts for delayed recharge (Bloomfield and Marchant, 2013). In previous studies, this correlation was used to demonstrate either the absence (Haas and Birk, 2017;Chu, 2018;Bloomfield et al, 2015;Kumar et al, 2016), or presence of human-influence on groundwater level observations (Lorenzo-Lacruz et al, 2017;Lee et al, 2018) . In this study, the presence or absence of human-influence on groundwater was determined in relation to the lowest SPI Q -SGI correlation of each near-natural reference cluster.…”

General response to Reviewer 1

“…There are two main reasons why the long term average, high SPI Q -SGI correlations may be reduced. The first reason for reduced long-term SPI Q -SGI correlation is when groundwater level response becomes disconnected from driving precipitation under confined conditions (Bloomfield, et al 2015;Lee, et al 2018). This is not considered to be a significant issue with the sites that we have investigated, as only a few sites are semi-confined (see notes in Table 1).…”