Characteristics of local groundwater recharge cycles in South African semi-arid hard rock terrains – rainwater input

E Van Wyk,D Vermeulen,Gj Van Tonder

doi:10.4314/wsa.v37i2.65860

Abstract

Rainfall events in semi-arid regions of South Africa are characteristically erratic in terms of depths and recurrence rates. Chemical assessment of cyclic rainwater has recognised 3 intervals, spaced over the hydrological cycle, reporting diverse hydrochemical compositions of rainwater in winter and summer rainfall regions. Winter rainwater is generated over the south Atlantic maritime waters. This rainwater contains noticeably higher concentrations of oceanic aerosols (NaCl) than the summer rainwater generated in the Intertropical Convergence Zone/equatorial western Indian Ocean. Sporadic addition of terrigenous dust generated over the continent substantially elevates concentrations of non-oceanic nitrogenous and sulphurous aerosols in the summer rainwater. Prominent seasonal variations in the rainwater hydrochemistry signature coincide with cyclic rainfall depths, characteristic of the semi-arid climate. Macro-element concentrations during the summer dry period, April to September, are relatively high in relation to those recorded for the wet cycle, October to March. However, the latter period reports a noticeably depleted hydrochemical rainwater input into the local groundwater budget during the peak rainfall period (January to March). The October-December (early) period represents a phase between a dusty, dry winter atmosphere and a relatively flushed atmosphere in December, after the first regional rainfall manifests around middle September. Individual early rainfalls contain even higher hydrochemical concentrations than the previous dry period, which subsequently diffuses as the airborne moisture content increases towards the peak rainfall period starting in January. Continuous rainfall event monitoring in the summer semi-arid regions identified short-term wet cycles contain ing extraordinary high rainfall events, referred to as episodic events. These wet cycles are highly erratic in time and may last from 3 to 8 consecutive days with a recurrence rate of 1 in 5 years. The rainwater hydrochemistry signature differs significantly from the normal rainfall composition and represents a unique opportunity for tracing the infiltrating rain water. For example, chloride concentrations from individual, high rainfall events (40 to 150 mm) may be as low as 0.4 mg·l-1, whereas the background value varies around 0.8 mg·l -1 . Environmental chloride represents a conservative tracer for estimating the migration between rainwater and groundwater recharge. The concentration levels are not constant throughout the year and may lead to erroneous assumptions when performing groundwater recharge estimations using accumulated rainwater samples and uncontrolled groundwater sampling techniques.

Highlights

Assessment of groundwater recharge in semi-arid regions requires high-level monitoring of a range of hydrometeorological variables, covering the hydrogeochemistry and rainfall patterns at groundwater recharge terrains, as well as the hydrochemistry of the rechargeproducing rainfall surplus reporting at the water table interface
The hydrological variables collated in this study focus on local, direct groundwater recharge events initiated by local short-term rainfall sequences
Dedicated observations of long-term rainfall patterns show that roughly 1 in 5 Southern African hydrological cycles (HC) contain a sequence of high-input rainfall events spaced over a high intensity rain-week period, which may total 250 to 300 mm

Summary

Introduction

Assessment of groundwater recharge in semi-arid regions requires high-level monitoring of a range of hydrometeorological variables, covering the hydrogeochemistry and rainfall patterns at groundwater recharge terrains (i.e. the fractured, hard rock terrains), as well as the hydrochemistry of the rechargeproducing rainfall surplus reporting at the water table interface. It illustrates the annual south-north migration of the 3 climate zones and the pathway of synoptic systems during the winter and summer seasons driving various rainfall events over Southern Africa. Summer rainfall regions The most prominent system dominating the Southern African atmosphere in summer time is the ITCZ, positioned across the equator around the mid-summer period (Decn to Febn+1, Fig. 1) This is a complex low-pressure synoptic system enhancing maritime evaporation over the warm, equatorial western Indian Ocean, and probably the eastern Atlantic Ocean, during periods with above-normal seawater temperatures (Summermax Synoptic Pattern, Fig. 1). Southern Africa’s weather pattern is dominated by the SHPZ This zone is characterised by 3 prominent high-pressure cells (anticyclones) situated across the sub-continent, i.e. the South Atlantic, the Botswana/Kalahari and the Indian Ocean Anticyclones. These values were included with the wet chloride input to calculate total environmental chloride deposition

Background

Utrecht

Conclusions and recommendations