An observation-based method to quantify the human influence on hydrological drought: upstream–downstream comparison

Abstract

In this present era of the Anthropocene, human activities affect hydrology and droughts. Quantifying this human influence improves our understanding and builds fundamental knowledge for water resource management. Analysis of observation data is useful in progressing this knowledge as these human activities and feedbacks are intrinsically included. Therefore, here we present an observationbased approach, the upstream-downstream comparison, to quantify changes in hydrological drought downstream of a human activity. We demonstrate this approach in a basin in northern Chile, where a reservoir was introduced. A sensitivity analysis is performed to assess how different choices of drought analysis threshold can affect the results and interpretation. We find that many commonly used choices do not exclude human activities from the threshold and therefore could be underestimating the change detected due to the human influence. The upstream-downstream comparison avoids this through the application of the upstream station threshold rather than the human-influenced downstream station.

Using paired catchments to quantify the human influence on hydrological droughts

“…For example, a oneto-one comparison of events (chronological pairing) is not recommended (Alila et al, 2009;Zégre et al, 2010); instead multiple characteristics like frequency and magnitude, taking into account all events in the period of record, should be analysed to prevent underestimation of effects. Here, we transferred the threshold from the benchmark catchment to the human-influenced catchment to make sure that the effect of the human activities is not included in the threshold, again preventing underestimation (Rangecroft et al, 2019). However, there might be reasons to look at anomalies relative to each catchment's own threshold, for example by using standardised drought indices.…”

“…For quantification of the human influence, the threshold needs to be generated from the benchmark catchment time series ( Fig. 2; Rangecroft et al, 2019), therefore excluding any influence of human activities on the threshold. This benchmark threshold is then applied to both catchments for the drought analysis (Fig.…”

“…2. The "upstream-downstream" approach (Rangecroft et al, 2019) compares hydrological droughts downstream of a disturbance with those upstream, which are assumed to be unaffected. This method uses observation data only and uncertainty comes from the possible non-linear relationship between the upstream and downstream gauging stations.…”

Anthropogenic activities alter drought termination

“…The Q 80 threshold for each catchment was calculated using the full length of the natural dataset, which was then used to identify drought events in both the natural and human-influenced data. This was important to effectively show the impact of human activity on hydrological droughts (Liu et al, 2016;Rangecroft et al, 2019) and their termination characteristics. Ideally, a time period of at least 30 years of data is used to establish the drought threshold (McKee et al, 1993).…”

“…This effect is visible in the Ariège and Bilina basins, where the artificial flow regime created by the reservoirs and water transfers (Firoz et al, 2018) alters the termination start and end months. Changes are highly pronounced in the Ariège catchment, however it is likely that the utility (hydropower vs irrigation or drinking water supply) and seasonal operation of the reservoir is an important factor in the resulting flow regime and consequent timings of downstream drought events (Rangecroft et al, 2019). Catchments influenced by abstractions tend to have naturally-existing patterns in their timing of termination amplified, seen especially in the Svitata and somewhat in the Upper-Guadiana, Thames and Lee, possibly relating to increasing drought occurrences during low-flow periods.…”