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
Authors: Roodari Artemis , Hrachowitz Markus , Hassanpour Farzad , Yaghoobzadeh Mostafa 
Published: April 2021

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.