Estimation of groundwater recharge via deuterium labelling in the semi-arid Cuvelai-Etosha Basin, Namibia

Abstract

The stable water isotope deuterium ((2)H) was applied as an artificial tracer ((2)H2O) in order to estimate groundwater recharge through the unsaturated zone and describe soil water movement in a semi-arid region of northern central Namibia. A particular focus of this study was to assess the spatiotemporal persistence of the tracer when applied in the field on a small scale under extreme climatic conditions and to propose a method to obtain estimates of recharge in data-scarce regions. At two natural sites that differ in vegetation cover, soil and geology, 500 ml of a 70% (2)H2O solution was irrigated onto water saturated plots. The displacement of the (2)H peak was analyzed 1 and 10 days after an artificial rain event of 20 mm as well as after the rainy season. Results show that it is possible to apply the peak displacement method for the estimation of groundwater recharge rates in semi-arid environments via deuterium labelling. Potential recharge for the rainy season 2013/2014 was calculated as 45 mm a(-1) at 5.6 m depth and 40 mm a(-1) at 0.9 m depth at the two studied sites, respectively. Under saturated conditions, the artificial rain events moved 2.1 and 0.5 m downwards, respectively. The tracer at the deep sand site (site 1) was found after the rainy season at 5.6 m depth, corresponding to a displacement of 3.2 m. This equals in an average travel velocity of 2.8 cm d(-1) during the rainy season at the first site. At the second location, the tracer peak was discovered at 0.9 m depth; displacement was found to be only 0.4 m equalling an average movement of 0.2 cm d(-1) through the unsaturated zone due to an underlying calcrete formation. Tracer recovery after one rainy season was found to be as low as 3.6% at site 1 and 1.9% at site 2. With an in situ measuring technique, a three-dimensional distribution of (2)H after the rainy season could be measured and visualized. This study comprises the first application of the peak displacement method using a deuterium labelling technique for the estimation of groundwater recharge in semi-arid regions. Deuterium proved to be a suitable tracer for studies within the soil-vegetation-atmosphere interface. The results of this study are relevant for the design of labelling experiments in the unsaturated zone of dry areas using (2)H2O as a tracer and obtaining estimations of groundwater recharge on a local scale. The presented methodology is particularly beneficial in data-scarce environments, where recharge pathways and mechanisms are poorly understood.

A deuterium-based labeling technique for the investigation of rooting depths, water uptake dynamics and unsaturated zone water transport in semiarid environments

“…Department of Soil, 2002) and iii) calibration (based on data published in Beyer et al, 2015) using the SVAT model DAISY (Abrahamsen and Hansen, 2000). Based on these parameter sets, the depth associated with stage I evaporation and diffusive flux over the dry season (stage II evaporation) were calculated.…”

“…While there might be a significant amount of water surpassing the root zone on site due to generally favorable conditions for infiltration during the rainy season (e.g. Beyer et al, 2015), this same water (or a fraction of it) might be lost in the subsequent dry season due to water vapor transport. If there was a water table at that depth, loss would be from there.…”

“…where L G [m] is the gravity characteristic length or length associated with stage I evaporation, In order to obtain an estimate of the length associated with stage I evaporation (in isotope hydrology it is also referred to as the zero flux plane), we applied equations 3 and 4 to our data assuming i) a dry soil (θ = 0.01, which has been observed on site; Beyer et al, 2015), ii) a temperature of 25 °C, iii) a water vapor density at surface ranging from 5 to 10 gm -3 (taken from estimations by the International Telecommunication Union, http://www.itu.int/) in June/July/August and iv) a vaporization plane at a constant depth. The results, together with soil hydraulic characteristics of the soil on site, are summarized in Table 2.…”

“…The results, together with soil hydraulic characteristics of the soil on site, are summarized in Table 2. We used data derived from soil cores, pore size distribution and fitted model parameters from a labeling experiment conducted by Beyer et al (2015) for comparison. Based on the fitted model (which is believed to be the best-estimate), a characteristic depth of ~22 cm (based on the calibrated parameters) is associated with Stage I evaporation.…”

In situ unsaturated zone water stable isotope (<sup>2</sup>H and <sup>18</sup>O) measurements in semi-arid environments: a soil water balance

“…Soil water stable isotope studies have also been used to reduce parameter uncertainty in unsaturated zone model approaches (Sprenger et al, 2015). Further work has been done to use soil water stable isotopes for quantitative recharge estimations (e.g., Beyer et al, 2015), which are actively discussed in earlier and recent reviews (e.g., AlPublished by Copernicus Publications on behalf of the European Geosciences Union.…”