Water Scarcity in the era of Climate Change

Increasing population has created a great pressure on food security and agricultural productivity. The rapid increase in population is causing a competition for land, water, energy and other resources that contribute in food production in the era of climate change.

By 2050 approximately 70% more food will have to be produced to feed growing populations, while climate change is estimated to have already reduced global yields of maize and wheat by 3.8% and 5.5%, respectively. The continuous use of chemical fertilizers, current farming practices and greenhouse gas emissions are severely affecting climate.

Present water shortage is one of the primary world issues and according to climate change projections, it will be more critical in the future. Since water availability and accessibility are the most significant constraining factors for crop production, addressing this issue is indispensable for areas affected by water scarcity.

Climate change will significantly impact agriculture by increasing water demand, limiting crop productivity and by reducing water availability in areas where irrigation is most needed or has comparative advantage. Global atmospheric temperature is predicted to rise by approximately 4 ˚C by 2080, consistent with a doubling of atmospheric CO₂ concentration.

Mean temperatures are expected to rise at a faster rate in the upper latitudes, with slower rates in equatorial regions. Mean temperature rise at altitude is expected to be higher than at sea level, resulting in intensification of convective precipitation and acceleration of snow melt and glacier retreat. In response to global warming, the hydrological cycle is expected to accelerate as rising temperatures increase the rate of evaporation from land and sea.

Thus rainfall is predicted to rise in the tropics and higher latitudes but decrease in the already dry semi-arid to arid mid-latitudes and in the interior of large continents.

Water-scarce areas of the world will generally become drier and hotter. Both rainfall and temperatures are predicted to become more variable, with a consequent higher incidence of droughts and floods, sometimes in the same place. Runoff patterns are harder to predict as they are governed by land use as well as uncertain changes in rainfall amounts and patterns. Substantial reductions (up to 40 percent) in regional runoff have been modelled in southeastern Punjab and in other areas where annual potential evapotranspiration exceeds rainfall.

Relatively small reductions in rainfall will translate into much larger reductions in runoff, for example, a 5 percent fall precipitation in southeastern Punjab will result in a 25 percent reduction in runoff. In glacier-fed river systems, the timing of flows will change, although mean annual runoff may be less affected. As temperature rises, the efficiency of photosynthesis increases to a maximum and then falls, while the rate of respiration continues to increase more or less up to the point that a plant dies.

All other things being equal, the productivity of vegetation thus declines once temperature exceeds an optimum. In general, plants are more sensitive to heat stress at specific (early) stages of growth, (sometimes over relatively short periods) than to seasonal average temperatures. Increased atmospheric temperature will extend the length of the growing season in the northern temperate zones, but will reduce it almost everywhere else. Coupled with increased rates of evapotranspiration, the potential yield and crop water productivity will fall.

However, because yields and water productivity are now low in many parts of the developing world, this does not necessarily mean that they will decline in the long term. Rather, farmers will have to make agronomic improvements to increase productivity from current levels. Increased atmospheric concentrations of CO₂ enhance photosynthetic efficiency and reduce rates of respiration, offsetting the loss of production potential due to temperature rise.

However, early evidence was obtained from plant level and growth chamber experiments and has not been corroborated by field-scale experiments; it has become clear that all factors of production need to be optimal to realize the benefits of CO₂ fertilization. Early hopes for substantial CO₂ mitigation of production losses due to global warming have been restrained.

A second line of reasoning is that by the time CO₂ levels have doubled, temperatures will also have risen by 4 ˚C, negating any benefit. Agriculture will also be impacted by more active storm systems, especially in the tropics, where cyclone activity is likely to intensify in line with increasing ocean temperatures.

Evidence for this intuitive conclusion is starting to emerge. Sea-level rise will affect drainage and water levels in coastal areas, particularly in low-lying deltas, and may result in saline intrusion into coastal aquifers and river estuaries.

Estimates of incremental water requirement to meet future demand for agricultural production under climate change vary from 40–100 percent of the extra water needed without global warming. The amount required as irrigation from ground or surface water depends on the modelling assumptions on the expansion of irrigated area between 45 and 125 million ha.

One consequence of greater future water demand and likely reductions in supply is that the emerging competition between the environment and agriculture for raw water and consequently the matching of supply and demand is harder to reconcile.

Conclusions

Water is a key resource for the development of any human activity. In many countries, the available water supply and the uneven distribution of these resources in time and space are pressing issues. It is projected that a large share of the world’s population, up to two-thirds, will be affected by water scarcity over the next several decades. The availability of water for farming is an essential condition for achieving satisfactory and profitable yields, both in terms of unit yields and quality.

The correlation between the expected increase in irrigation water requirements, critical values of renewable freshwater resources and economic water scarcity, indicates the necessity for regional policy coordination and careful water management strategies at the national and site levels. Such policy coordination and water management strategies could avail themselves of scientific research that should actively involve in dealing with water scarcity.

Currently, there are many studies on strategies and policies for water supply management, bio-molecular and genetic research to find more drought-tolerant cultivars, on climate change and its impact on future irrigation requirements and yield and on climate adaptation strategies.

More investments in infrastructure development (i.e., dams and water supply pipe networks) would help future populations to cope with the growing water demand and where an uneven distribution of precipitation in time is expected.

These investments are especially needed in those countries affected, or projected to be affected, by water scarcity. The application of efficient water management strategies is a key element to increase water productivity.

In addition to the assessment of crop management strategies, the improvement of irrigation systems and irrigation schemes can lead to a more efficient and sustainable agricultural water management. In addition, models may serve as a decision support tool for regional and on-farm system management to develop strategy scenarios for sustainable farming systems.

Authors:  Muhammad Nazim¹, Dr. Shazia Anjum² and Qurat-Ul-Ain Sadiq³

¹Department of Agronomy, MNS-University of Agriculture Multan, Pakistan.

²Cholistan Institute of Desert Studies, The Islamia University of Bahawalpur, Pakistan

³Department of Soil and Environmental Sciences, MNS-University of Agriculture, Multan, Pakistan.