Signatures of human intervention – or not? Downstream intensification of hydrological drought along a large Central Asian river: the individual roles of climate variability and land use change

Abstract

Abstract. The transboundary Helmand River basin (HRB) is the main drainage system for large parts of Afghanistan and the Sistan region of Iran. Due to the reliance of this arid region on water from the Helmand River, a better understanding of hydrological-drought pattern and the underlying drivers in the region is critically required for effective management of the available water. The objective of this paper is therefore to analyze and quantify spatiotemporal pattern of drought and the underlying processes in the study region. More specifically we test for the Helmand River basin the following hypotheses for the 1970–2006 period: (1) drought characteristics, including frequency and severity, systematically changed over the study period; (2) the spatial pattern and processes of drought propagation through the Helmand River basin also changed; and (3) the relative roles of climate variability and human influence on changes in hydrological droughts can be quantified. It was found that drought characteristics varied throughout the study period but largely showed no systematic trends. The same was observed for the time series of drought indices SPI (standard precipitation index) and SPEI (standardized precipitation evapotranspiration index), which exhibited considerable spatial coherence and synchronicity throughout the basin, indicating that, overall, droughts similarly affect the entire HRB with few regional or local differences. In contrast, analysis of the SDI (streamflow drought index) exhibited significant negative trends in the lower parts of the basin, indicating an intensification of hydrological droughts. It could be shown that with a mean annual precipitation of ∼ 250 mm yr−1, streamflow deficits and thus hydrological drought throughout the HRB are largely controlled by precipitation deficits, whose annual anomalies on average account for ±50 mm yr−1, or ∼ 20 % of the water balance of the HRB, while anomalies of total evaporative fluxes on average only account for ±20 mm yr−1. Assuming no changes in the reservoir management practices over the study period, the results suggest that the two reservoirs in the HRB only played a minor role for the downstream propagation of streamflow deficits, as indicated by the mean difference between inflow and outflow during drought periods, which did not exceed ∼ 0.5 % of the water balance of the HRB. Irrigation water abstraction had a similarly limited effect on the magnitude of streamflow deficits, accounting for ∼ 10 % of the water balance of the HRB. However, the downstream parts of the HRB moderated the further propagation of streamflow deficits and associated droughts because of the minor effects of reservoir operation and very limited agricultural water in the early decades of the study period. This drought moderation function of the lower basin was gradually and systematically inverted by the end of the study period, when the lower basin eventually amplified the downstream propagation of flow deficits and droughts. Our results provide plausible evidence that this shift from drought moderation to drought amplification in the lower basin is likely a consequence of increased agricultural activity and the associated increases in irrigation water demand, from ∼ 13 mm yr−1 at the beginning of the study period to ∼ 23 mm yr−1 at the end, and thus in spite of being only a minor fraction of the water balance. Overall the results of this study illustrate that flow deficits and the associated droughts in the HRB clearly reflect the dynamic interplay between temporally varying regional differences in hydro-meteorological variables together with subtle and temporally varying effects linked to direct human intervention.

Assessment of the Trust Crisis between Upstream and Downstream States of the Helmand River Basin (1973–2022): A Half-Century of Optimism or Cynicism?

“…In summary, accusations of dishonesty and a lack of accountability have been widely made by authorities and officials from both parties, especially during the past 26 official meetings of the commisioners, ,,,, as well as by scholars from both countries. − ,− ,, We, however, argue that honesty and accountability, which are the first stages in developing trust between the riparian nations of a shared watercourse, could be achieved in the Helmand River Basin if both countries complied with the existing treaty by specifying the three points of water delivery at the political border of Afghanistan and Iran, as well as by weatherizing and upgrading the monitoring technology at the Dehrawud Station.…”

“…Furthermore, when the Iranian government demanded their rightful share of water from the Helmand River, especially during periods of drought, Afghan authorities claimed that the flow of the Helmand had declined due to climatic changes. 37 According to Nagheeby, 125 46 the commissioners of both riparian countries are accountable for specifying the three water delivery points near the Afghan−Iran political border after the treaty entered into force. 46 These water delivery points are (1) "at the place where the boundary line crosses the rude Seistan" and (2) "at two other places where the boundary line is located in the bed of the Helmand River between boundary pillars numbers 51 and 52" (also called markers 51 and 52) 43 (Article IIIa, pp 1−2).…”

Extreme low‐flow effects on riverine fauna: A perspective on methodological assessments

“…The authors found that catchment topography, precipitation and geology (in decreasing order) were the most important environmental characteristics governing the severity of ELFs. Similarly, Roodari et al (2021) reported that the downstream intensification of ELFs along the Helmand River, Afghanistan, was largely attributable to precipitation deficits, while flow regulation and irrigation abstraction practices yielded limited effects. Such large-scale studies are challenging to undertake as they require extensive data collation, quality assurance, and analysis of environmental information from multiple data sources (e.g.…”

Future changes in annual, seasonal and monthly runoff signatures in contrasting Alpine catchments in Austria

“…The model is based on hydrological response units (HRUs), as utilized and described, for example, by Gao et al (2014) and Prenner et al (2018). The aim is to represent dominant physical processes in the catchment based on topography and land cover classes while limiting model complexity (Savenije, 2010). A detailed model description is given in Sect.…”