Global water resources affected by human interventions and climate change

Abstract

Humans directly change the dynamics of the water cycle through dams constructed for water storage, and through water withdrawals for industrial, agricultural, or domestic purposes. Climate change is expected to additionally affect water supply and demand. Here, analyses of climate change and direct human impacts on the terrestrial water cycle are presented and compared using a multimodel approach. Seven global hydrological models have been forced with multiple climate projections, and with and without taking into account impacts of human interventions such as dams and water withdrawals on the hydrological cycle. Model results are analyzed for different levels of global warming, allowing for analyses in line with temperature targets for climate change mitigation. The results indicate that direct human impacts on the water cycle in some regions, e.g., parts of Asia and in the western United States, are of the same order of magnitude, or even exceed impacts to be expected for moderate levels of global warming (+2 K). Despite some spread in model projections, irrigation water consumption is generally projected to increase with higher global mean temperatures. Irrigation water scarcity is particularly large in parts of southern and eastern Asia, and is expected to become even larger in the future.

Keywords: ISI-MIP; WaterMIP.

Fig. 1.

Comparison of human impact and climate change effects on runoff at the river basin level. Basin averaged runoff values are calculated based on simulated discharge at the outlet of the river basins, and the median ensemble results are shown. (A) Control period (1971–2000) human impact simulations compared with control period naturalized simulations. (B) Basin averaged naturalized runoff for 2 K GMT increase, compared with control period naturalized simulations. (C) Basin averaged human impact runoff for 2 K GMT, compared with control period naturalized simulations.

Fig. 2.

(A) The difference between the absolute values in Fig. 1 A and B in basins where the human impact and climate signals are opposite, i.e., naturalized runoff increases. (B) The differences between the absolute values in Fig. 1 A and B in basins where both the climate signal and human impact signal are negative, i.e., runoff decreases. The red and yellow colors indicate that the control period human impacts are larger than future climate effects on naturalized runoff.

Fig. 3.

Box plots of relative changes in runoff for (A) the world, (B) Colorado, (C) Mississippi, (D) Nile, (E) Euphrates-Tigris, (F) Indus, and (G) Huang He for the control period (C) (1971–2000), 2 and 3 K GMT increases. The boxes illustrate the 25th, 50th, and 75th percentiles of the ensemble (47 members). The whiskers represent the total sample spread, and in addition the 5th and 95th percentiles are marked. The human impact results (orange bars) are compared with the naturalized simulations during the same time period, e.g., 2 K human impacts are compared with 2 K naturalized simulations. All climate and combined effects (blue and green bars) are compared with the control period naturalized simulations.

Fig. 4.

Irrigation water consumption and cumulative abstraction-to-demand (CAD) ratio at the grid cell level. (A) Ensemble median potential irrigation water consumption, control period (1971–2000). Light gray color represents areas where there is no, or very little, irrigation. (B) Ensemble median CAD, control period. (C) Differences in CAD between the control period and the 2 K GMT increase period. Negative numbers mean the CAD ratio decreases.

Fig. 5.

Ensemble statistics on irrigation water consumption for the control period (C) (1971–2000), 2 and 3 K GMT increases for (A) the world, (B) United States, (C) southwest Europe (here comprising Portugal, Spain, and France, (D) Pakistan, (E) India, and (F) China. The upper panels show annual potential and actual irrigation water consumption. The lower panels show CAD, i.e., the relationship between the actual and potential irrigation water consumption. The boxes illustrate the 25th, 50th, and 75th percentiles of the ensemble. The whiskers represent the total sample spread, and in addition the 5th and 95th percentiles are marked.