Impacts of climate change on water availability in Botswana

[摘要] The information in this report, along with the accompanying data sets are designed to assist with the development of a climate change action plan for Botswana. In this report, we derive the range of projected changes in rainfall, areal potential evapotranspiration, runoff and groundwater recharge due to 1 °C of global warming (corresponding to ~2030 under the A1B emission scenario). This range of results is presented as a ‘wet’, ‘median’ and ‘dry’ future projection, as derived from the outputs from 15 IPCC AR4 global climate models.Using the TRMM rainfall product and the NCEP potential evapotranspiration reanalysis, Botswana’s average historical rainfall is 485 mm and potential evapotranspiration is 1645 mm. After calibrating the Sacramento rainfall-runoff model to historical streamflow and regionalising the results across the whole country, areal average historical runoff is estimated to be 16 mm. Using the WAVES model, historical diffuse groundwater recharge is estimated to be less than a few mm per year over most of the country, with just a few areas in the far north with appreciable rates (30 mm/year) of recharge.Botswana straddles the tropic of Capricorn, where the outputs of global climate models are typically mixed, with some projecting a drier future climate, and some a wetter future climate. However, for Botswana there is more agreement that rainfall will decrease in the future, with 10 of the 15 global climate models used in this study projecting a decrease in mean annual rainfall in the future and only 5 projecting an increase. Averaged across the country, the dry future projection is for a reduction in rainfall of 50 mm (10%), the median projection is for a reduction of 15 mm (3%), and the wet projection is for an increase of 13 mm (also around 3%). Across the dry season, all global climate models project a decrease in rainfall.Due to the low rates of historical groundwater recharge and the relatively small projected changes in rainfall, changes in diffuse groundwater recharge by ~2030 are projected to be fairly insignificant. Over the far north of the country, however, recharge may decrease by ~50%, or it may increase by ~50%.For potential evapotranspiration, there is more agreement with all 15 GCMs projecting an increase by ~2030. Averaged across the country, these increases range from 20 mm (1%) up to 60 mm (3%) with the median result being an increase of 35 mm (2%). These projected changes in rainfall and potential evapotranspiration combine to influence projected changes in runoff. Averaged across the country, under the dry future projection, runoff is projected to decrease by 5 mm (30%); under the median projection, runoff is projected to decrease by 2 mm (12%); while under the wet future projection, runoff is projected to increase by just 1 mm (6%). In the dry season, all projections are for a decrease in runoff, however historical runoff is fairly small at this time of the year in any case, with most rivers ceasing to flow through the dry season. This cease to flow may therefore occur earlier and last longer by ~2030 due to the impacts of climate change.The results presented in this report are only valid for those rivers that are sourced from within Botswana. The impact of climate change on large rivers which are sourced outside Botswana (such as the Okavango) would require a far more extensive study, collecting data from all neighbouring countries which contribute to the flow in these rivers. Nevertheless, the outputs of this study indicate that a drying trend is the most likely projected outcome for Botswana by ~2030.The data underlying this report are available to any person requesting it. In particular, these data should be of use when determining the impact of climate change on the water resources, agriculture and ecosystems of Botswana.

[发布日期] 2012-02-01 [发布机构] CSIRO

[学科分类] 地球科学(综合)