Global water resources and ecosystems

In Fig. 4.1, the long-term average mean annual runoff (MAR) of rivers is shown. This is calculated using a global runoff model (Smakhtin, VU., Revenga, C. and Doll, P, unpublished). Between regions there is a large difference in runoff. The dark areas show high runoff and the light areas low runoff. As can be expected, high yearly runoff occurs in the tropics and the yearly runoff in the desert regions is very low. The centre of North America, Australia and central Asia are places in the world where the water resources are scarce. These are the potential hotspots when CC leads to a reduction in runoff. Ecosystems need a certain amount of water to subsist and this amount varies between regions and ecosystems. For the majority of the rivers in the world there is no recommendation on the environmental water requirement. To estimate the amount of water that ecosystems need worldwide, a modelling study is performed (Smakhtin, VU., Revenga, C. and Doll, P, unpublished). This study used the environmental water requirement as the definition of the amount of water needed to sustain ecosystems.

The environmental water requirement (EWR) is calculated by adding the

Mapa Mundi Vectorial
Fig. 4.1. Map of long-term average annual water resources in mm/year (Smakhtin etal., 2003).

environmental low flow requirement (LFR) to the environmental high flow requirement (HFR). The LFR of a river is defined as the average runoff that is exceeded 90% of the time throughout the year, in other words, during 10% of the year runoff is lower. This information can be extracted from a flow duration curve of a river as shown in Fig. 4.4. The HFR is also defined as the discharge exceeded during a certain percentage of time a year. This percentage varies between different types of rivers. In a highly variable regime, the impact of changes in high flow on the system will be more severe than in a more stable system. For rivers with a highly variable flow, the HFR is equal to the runoff that is exceeded 20% of the time on average throughout the year. For rivers with a very stable flow, the HFR is considered to be zero.

Defining the EWR as above is a first step of estimating the health of ecosystems in relation to water availability. It is a very general method, and it does not take into account a desired state of ecosystems in the basin or an environmental management class in which an ecosystem needs to be maintained. That was not feasible in this first global-scale assessment of environmental water requirements (Smakhtin, VU., Revenga, C. and Doll, P., unpublished).

The global EWR projections and calculations show that globally 20-50% of the mean annual river runoff is needed to sustain ecosystems in the current status. The light coloured areas in Fig. 4.2 represent the basins where a small proportion of the runoff is needed for sustaining the ecosystems and in the dark areas up to 50% of the runoff is needed for sustaining the ecosystems. These estimations could be on the low side because of the assumptions made in the model (Doll et al., 2003). For instance, the sensitivity and importance of the (aquatic) ecosystem were not taken into account. When an ecosystem is very important, a larger proportion of the water could be allocated to it.

When ecosystems are regarded as a water user, a large proportion of the water resources should be allocated to them. Hence, knowing that human water withdrawals

Fig. 4.2. Environmental water requirements for sustaining ecosystems as percentage of total runoff (Smakhtin et al., 2003).

increase and that climate change may decrease water availability, further strains on the water resources in the basin can be expected.

By combining EWR and water availability, a Water Stress Indicator (WSI) can be calculated as shown in Fig. 4.3. The WSI is the water withdrawal (the EWR) as a proportion of water availability. This is shown in Fig. 4.3, where water stress in basins is presented, accounting for the environmental water requirements as shown in Fig. 4.2. The basins with a WSI higher than 0.7 are basins where there is not enough water to meet the environmental water requirements. Figures 4.2 and 4.3 are based on values under the current climate. It is expected these figures will change under influence of climate change and in the future more problems will exist between minimal environmental requirements and human use of water and the available water. In the marked basins, the water stress increases in comparison to the traditional water stress indicators, water withdrawal as a proportion of the average total water resource. In the dark areas the current WSI is very high and under CC this stress will probably increase, for instance around the Mediterranean where the precipitation is expected to decrease under CC (IPCC, 2001) and mid- and west North America.

Change in mean annual water availability is not the only quantitative indicator that measures stress to ecosystems. One can also look more specifically at low flows and high flows, since ecosystems are largely influenced by the occurrence of periods of low flows and high flows. These features can be similarly used as indicators for presenting environmental stress in river basins and, especially for low flow hydrology, much work has been done (Caruso, 2001; Smakhtin, 2001). Low flows can vary over the years and are influenced by precipitation, snow melt and anthropogenic impacts, like groundwater abstraction, artificial drainage, changes in vegetation, deforestation, discharge from storage and water extraction for industry, agriculture and municipal use. They are

Major river basins

Fig. 4.3. A map of water stress indicators that takes into account environmental water requirements in river basins. The circles indicate case studies used by Smakhtin et al. (2003).

Major river basins

Fig. 4.3. A map of water stress indicators that takes into account environmental water requirements in river basins. The circles indicate case studies used by Smakhtin et al. (2003).

important since ecosystems need a minimum discharge to ensure fish passage, maintain certain temperature levels, habitat maintenance, sedimentation control, etc. Some ecosystems also need a high flow for a yearly flooding, like floodplains ecosystems.

A so-called flow duration curve shows how to address changes in low flow and high flow (Fig. 4.4). This curve displays river discharge at a certain point along the river, in relation to the percentage of time a certain discharge has occurred. In Fig. 4.4 there are two curves of a river; the continuous line represents the current situation; the interrupted line shows the future situation under climate change. The discharge that occurs under the current circumstances for 100% of time is the value on the right side of the graph. In this case the minimum discharge is approximately 1600 m3/s. Towards the left of the graph, the probability that the discharge is exceeded becomes lower, in other words it occurs less frequently during the year. For instance, the chance that a discharge of 3500 m3/s is exceeded is 35%. The probability of exceeding a discharge decreases with an increase in discharge. This continues until the probability is zero, which is the maximum discharge of the river. The maximum discharge of this river is 8000 m3/s. From the graph, low and high flow discharges can be derived. When the minimum and maximum discharge requirements for the ecosystems in the basin are known, these can be compared with the flow duration curve of the river. The interrupted line in Fig. 4.4, the future situation under climate change, shows a shift in the minimum discharge of the river to 500 m3/s. When, for instance, the minimum flow requirement of the ecosystem is 1000 m3/s, this is no longer met in the future. This information is important to a water manager of the basin. The management of reservoirs can be changed to increase the instream of water to the river through reservoir releases during extreme low flow periods. Hence, low flow requirements can be met in the basin. If the changes in low flow are allowed, these can negatively influence the habitat dynamics of a system and in the long term can change the ecosystems (Dakova et al., 2000).

Probability of exceedence P(%) Fig. 4.4. Example of a flow duration curve.

Regional water resources and aquatic ecosystems

Global changes in water resources described earlier in this chapter can be refined on a basin scale. A basin can be regarded as a natural hydrological unit, which contains the total water system within its borders and has different users that are all dependent on the same water resource. There is no exchange with other basins or resources, so the basin can be seen as a whole. On this regional scale it is useful to make a distinction between aquatic and terrestrial ecosystems, because there is a difference between the issues and expected climate change effects they have to cope with. The impacts described in this paragraph are possible effects, which obviously vary between different regions and ecosystems.

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