Abstract
Drought phenomena are attributed to water availability deficit that is caused by low precipitation. However, droughts are quite complex and cannot simply be defined on the basis of precipitation as other factors may have an influence. In this study, we investigated the spatio-temporal patterns of droughts in Lake Chilwa Basin, an endorheic lake basin that has recently experienced major recurrent lake recessions. The standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI) at 6- and 12-month timescales were used to evaluate drought severity variations from 1970 to 2018, in relation to the recessions. The stationarity difference in rainfall between 1973 to 1995 and 1996 to 2018 and climatological trends were tested using Mann-Whitney and Mann-Kendall tests, respectively. The El Niño Southern Oscilation (ENSO) influence on rainfall was also investigated. In general, the results show a statistically insignificant decreasing rainfall trend, coupled with statistically significant temperature increase (α=0.05). In addition, both indices broadly detected droughts within similar category ranges and variation patterns, suggesting minimal influence of temperature on droughts compared to rainfall. The study also reveals that not every ENSO event leads to low rainfall in the basin. It is further shown that unlike past major recessions, e.g., 1994/95, recent lake dry-ups of 2012 and 2015 were as a result of milder droughts. Moreover, the trigger threshold of lake dry-ups is shown to have shifted; such that average annual rainfall below 1000mm is likely to yield a dry-up in recent times than before, which may be attributable to anthropogenic pressure.
Highlights
Drought phenomena are associated with water availability deficit that is caused by low precipitation compared to a long term average (Dubrovsky et al, 2009)
3.0 Methods and data The standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI) were applied to evaluate the spatio-temporal pattern of drought in the Lake Chilwa Basin from the year 1970 to 2018, a period that includes some of the most severe droughts historically recorded in the Chilwa Basin leading to complete lake dry-ups
Rainfall in the Lake Chilwa basin displayed a decreasing tendency such decrease was not generally significant based on MK test at alpha level (α) of 0.05
Summary
Drought phenomena are associated with water availability deficit that is caused by low precipitation compared to a long term average (Dubrovsky et al, 2009). Drought events are quite complex (Vicente-serrano, 2007) and cannot be defined on the basis of precipitation as other factors such as evapotranspiration and soil moisture may have an influence Due to their complex characteristics, droughts can be categorised as either agricultural, meteorological, hydrological or socioeconomic depending on their specific area of impact and driving factors (Leng et al, 2015). What is always clear though is that droughts are costly and damaging to societies and ecosystems more than any other natural disaster (Sheffield et al, 2009; Wang et al, 2015; Wilhite, 2000) especially when an occurrence leads to extreme low flows in rivers and abnormal low levels in lakes, reservoirs, and groundwater (Loon, 2015) Irrespective of their impacts, droughts do not receive much of publicity and scientific attention as compared to other disasters such as floods, often because they are not as dramatic (Loon, 2015). Droughts generally develop slowly and are considered a “creeping disaster” which makes their onset and cessation hard to notice
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