Pan evaporation and wind run decline in the Cape Floristic Region of South Africa (1974–2005): implications for vegetation responses to climate change

Abstract

In many regions of the world, increasing temperatures in recent decades are paradoxically associated with declining pan evaporation, but evidence is sparse for this trend from the southern hemisphere in general and sub-Saharan Africa in particular. In this study, we examined changes in pan evaporation and four other meteorological variables (rainfall, wind run, temperature and vapour pressure deficit) at 20 climate stations in the predominantly winter-rainfall Cape Floristic Region (CFR) of South Africa over the period 1974-2005. Our results show that pan evaporation has declined significantly at 16 climate stations at an average rate of 9.1 mm a −2 while wind run has declined significantly at all climate stations by more than 25% over the study period. Annual rainfall has not changed significantly at any of the climate stations while maximum temperature has increased significantly at all but one climate station at an average rate of 0.03 • C a. −1 over the study period. The trends in vapour pressure deficit are mixed and no clear regional pattern is evident. Our results raise important questions about the predicted catastrophic impact that the projected changes in twenty-first century climates will have on the rich flora of the region. If evaporative demand has declined over the last 30 years in the Cape Floristic Region then it is possible that more water has become available for plant growth, infiltration and runoff despite the widespread increase in temperature. However, decreased pan evaporation and wind run combined with increased temperatures could potentially reduce transpiration and exacerbate heat stress of plants on increasingly frequent 438 Climatic Change (2011) 109:437-452 hot and windless days during the summer drought. Contrary to other predictions for the area, it is also likely that the changing conditions will decrease the frequency and/or intensity of fires which are an important component of the ecology of the fireadapted CFR. Consideration of other factors besides changes in temperature and rainfall are essential in debates on the impact of climate change on the vegetation of this region.

Spatiotemporal variation and driving forces of reference evapotranspiration in Jing River Basin, northwest China

“…China as a whole also shows a decreasing trend for ET o and pan evaporation (Gao et al ., ; Yang and Yang, ). Many decreasing trends have also been found in Thailand (Limjirakan and Limsakul, ), northeast India (Jhajharia et al ., ) and Tons River Basin (Darshana et al ., ) in India, Cape Floristic Region of South Africa (Hoffman et al ., ), and the Platte River Basin, USA (Irmak et al ., ). The decreasing magnitude of annual ET o in Jing River Basin is lower compared with other regions of China and other countries (Table S1 in Supporting Information).…”

“…Wind speed has significantly decreased from 1960 to 2005 in Jing River Basin, which has also been observed globally, such as North America, Europe, Australia, China, South Africa, and India (Yin et al ., ; Hoffman et al ., ; Darshana et al ., ; Jhajharia et al ., ). Weak atmospheric circulation has been suggested as the reason for a decrease in wind speed (Zhang et al ., , ; Zhang et al ., ; Roderick et al ., ).…”

Recent trends in the climate of Namaqualand, a megadiverse arid region of South Africa

“…60 There are no previous studies on the trends in evapotranspiration for the Succulent Karoo or the Namaqualand region. In one of the only related studies, Hoffman et al 61 found that pan evaporation declined significantly at an average rate of 9.1 mm per annum at 16 weather stations located in the Cape Floristic Region of South Africa to the south of Namaqualand as a result of a decline in wind run.…”

Intensifying postfire weather and biological invasion drive species loss in a Mediterranean-type biodiversity hotspot

“…Thus, plants are encountering higher temperatures and longer hot and dry spells during summer, which we expect to result in lower germination rates and higher mortality of seedlings or sprouting individuals in the first summer after fire. Although reduced wind run (lower wind speeds and greater numbers of still days) may reduce potential evapotranspiration and drought stress, it also reduces sensible heat loss, increasing the potential for heat stress (25).…”