Groundwater Resources Exploitation And Protection In Okavango Delta Under The Condition Of Climate Change

Okavango delta, the largest and the most primitive inland delta in the world, is located in the north of Botswana-an arid and semi-arid country in the Southern Africa. There is an obvious contradiction between water supply and demand in Botswana, where groundwater resources are important water sources or even the only effective source to maintain residents'life and production in most areas. The abundant groundwater of Okavango delta maintains the basic living and industrial and agricultural development of local residents, which is the indispensable and important wetland resource. However, under the background of global warming and the increasing human activities, the contradiction between groundwater supply and demand may be intensified and the groundwater may be worsened in this delta because it is also located in a sensitive area and fragile zone of global change. Therefore, it is very necessary to carry out the research on the development and utilization of groundwater resources in Okavango delta and very important for prevention and control of groundwater pollution. This will further enhance China-Africa friendship.There is a direct hydrological link between the groundwater and long-term or seasonal wetland water in Okavango delta, where groundwater recharge and flow are both controlled by wetlands. The study of the impacts of climate change on wetlands can well reflect the impacts of climate change effects on groundwater. Therefore, this paper takes "surface water-wetland-groundwater "as the main line and analyzes the characteristics of climate change over many years with climate change data from previous years in Okavango delta. Remote sensing interpretation method is used to reveal the impacts of climate change on the delta wetlands and groundwater, and hydrochemistry and isotope methods are used to analyze the distribution and hydro-chemical characteristics of surface water and groundwater. In conclusion, this study put forward problems that we may face in the exploitation of groundwater in Okavango delta and some Problem-solution approach and measures.The main contents and results of this research are as follows:1. Annual mean air temperature anomaly in Okavango delta shows an increasing trend in 1965-2008, and the climate tendency rate is about 0.37℃/10a. Annual mean precipitation anomaly and annual precipitation anomaly changed greatly, the correlation coefficients of annual mean precipitation anomaly and annual precipitation anomaly are not high and showed not obvious trend in the rise and fall. At present time, annual mean precipitation is plus anomaly year, and 5 years moving average shows a fluctuating increase in average rainfall years. In 1970-2008 periods, annual mean evaporation and temporal series showed weak correlation. Annual mean evaporation changed from plus anomaly year to minus anomaly year. Annual mean flow rate of Mohembo river showed downward trend. Year anomaly analysis showed the change rate of river flow is 63.52m3/s/10a.2. The impact of air temperature on nudation is largest. The correlation coefficient of air temperature and nudation is 0.986. The next is water body/herbaceous swamp, the correlation coefficient is 0.934. As the temperature increases, the increasing trend of nudation is the most obvious. The increasing temperature leads to the lowering of standing water depth in water body/herbaceous swamp, and then water body/herbaceous swamp gradually changed to non-wetland types. At the same time, the increasing temperature can also reduce groundwater recharge, elevate groundwater recharge temperature, and causes the change of groundwater hydrochemistry. Evaporation had largest impacts on woodland and sparse grassland, the correlation coefficients are 0.987 and -0.886. The rising evaporation can reduce soil moisture content, impact the growth of vegetation, and leads to woodland translates to sparse grassland. Strong evaporation can also cause surface water and phreatic water to occur evaporating concentration, salinity increasing, surface salinity increasing, saline and alkaline land increased. Rainfall had great impacts on the majority of land use types in delta, the correlation coefficients are between 0.65-0.85. Rainfall had the largest impacts on swamp meadow, the correlation coefficient is 0.829. With the increase of rainfall, swamp meadow showed obvious increasing trend. Changes of rainfall and rainfall distribution may cause local change in groundwater recharge and runoff. River flow and water body/herb marsh, swamp meadow, saline and alkaline land was positively correlated. River flow and grassland, woodland, sparse grassland was negatively correlated. So when the source of Okavango river Angola had abundant rainfall, rainfall increases, and human activities didn't increase in the upper reaches of the river, grassland, woodland and sparse grassland will transfer to wetland types. When wetland area increasing, there is a corresponding increase of the saline and alkaline land.3. The interior of Okavango delta is mostly permanent and seasonal wetlands, the groundwater level is close to the surface. Surface water and groundwater had strong interaction. Groundwater is mostly HCO3 type, salinity is generally lower than.5g/l. And the water saturation index of mineralization and dolomite are not saturated, the main hydrochemical process is mineral dissolution. The edge transition zone of delta, the groundwater level decreased gradually. The hydrochemistry type is from HCO3 type to SO4 type and Cl type, the main type is mixed. The salinity is 1～3g/l. The mineral solubility of groundwater is gradually decreased. The groundwater table of river drainage area and south of Thamalakane fault is deeper. Groundwater is mainly Cl type and the salinity is generally high. The salinity of some water samples are higher than 3g/l. the index of calcite and dolomite is close to saturate in most water samples. The main chemical process is evaporation concentration.Most of water samples in Okavango delta are modern water, the adjustment for age as 14C of only OW-3-G water sample is 3047.96a. The adjustment for age as 14C of water samples in South of Thamalakane fault is higher than Okavango delta. he adjustment for age as 14C of OW-19-G is the oldest,12606a.8D in groundwater in research area varied from -63.1‰to 0.6‰.δ18O varied from -9.33‰to 1.91‰. Hydrogen and oxygen isotopes relatively higher concentrated in downstream of Okavango delta than upstream. This indicates that the groundwater evaporation in downstream was larger than upstream, but was blocked in Thamalakane fault, not through the fault. Groundwater in Okavango delta was strong cyclic alternated with modern precipitation, surface water. The groundwater age was old in the north of fault, didn't accept modern atmospheric precipitation and modern groundwater recharge in south of fault, or accept supplies earlier than delta. Isotope data also proved that the East African Rift Valley and Thamalakane fault had hindered groundwater flow. Groundwater flow will limit the groundwater flow the central Botswana, and lead to uneven distribution of groundwater in Botswana.4. With the climate warming and increased human activity, Okavango delta may face problems that wetland will shrink, groundwater recharge will reduce, salinity of groundwater will increase, and other problems like groundwater pollution. Through carrying out a unified evaluation to groundwater, planning and management, dynamic monitoring surface water and groundwater level, and identify controlling the key water level, strengthening management of groundwater quality.