Land atmosphere feedbacks and their role in the water resources of the Ganges basin

https://doi.org/10.1016/j.scitotenv.2013.03.016Get rights and content

Highlights

  • The Ganges and Indus basins contain one of the largest contiguous areas of irrigation in the world.

  • The processes and magnitude of feedbacks between the land surface and the atmosphere depend on spatial scale.

  • Feedbacks into the Planetary Boundary Layer from irrigated land will reduce evaporation by as much as 31%.

  • Increased cloud amount due to evaporation by irrigated areas may reduce evaporation by between 5 and 10%.

  • Approximately 40% of the precipitation falling in the Himalayas originates from the irrigated areas in northern India and Pakistan.

Abstract

The northern Indian subcontinent has frequently been identified as a hotspot for land atmosphere interactions. It is also a region with the highest concentration of irrigated land and highest (and increasing) population density in the world. The available water in the region with which to grow food depends on the Asian monsoon, groundwater and melt from Himalayan snows. Any changes or disruptions to these sources of water could threaten the food supply. It is therefore essential to understand how the land surface, and in particular irrigated land, interacts with the atmosphere. It is anticipated that the interactions will occur on many scales. To an extent the magnitude and form of these will depend on the depth of the atmosphere which is affected. Thus at the local, or micro, scale it is the surface layer (some 10s m deep) which is cooled and moistened by the evaporation of irrigated water, at the meso-scale the Planetary boundary layer (up to 1 or 2 km) will be modified — with possible atmospheric moistening, increased cloud and rain formation and at very large scales the whole dynamics of the south Asian Monsoon will be affected. This illustrates a strong interaction between the Asian monsoon and the regional topography. Of considerable significance is the finding in this paper that up to 60% of the evaporation from irrigated areas in the summer months is ultimately recycled to Himalayan rainfall and so feedbacks to river flows in the Ganges.

Introduction

The Ganges Basin is home to almost one half a billion people and is one of the most densely populated regions of the world. The majority of the population rely on agricultural production in the region and 75% are rural populations. Currently agricultural production in the basin broadly balances consumption, although increasing population, increasing wealth and climate change are likely to push this balance into deficit in the coming decades. About one half of the total area of the Ganges basin is under cultivation and of this one third (approximately 35 m ha) is irrigated. The largest contiguous areas of irrigation (with over 75% of the land area irrigated) are found in north India and Pakistan along the Ganges and Indus rivers (see e.g. Siebert et al., 2005). In the states of Haryana and Uttar Pradesh, for example, over 50% of the total geographical areas of the states are under irrigation, (http://www.fao.org/nr/water/aquastat/irrigationmap/in/index.stm). A recent study Biemans et al. (2013--this issue) suggests only 44% of the irrigation demand in the Ganges can be met by surface waters and the remainder is extracted from other sources, mainly deep groundwater. This has led to a widely observed decrease in regional ground water levels (e.g. Rodell et al., 2009, Wada et al., 2010). Only 10% of the flow of the Ganges is supplied by the melt from the snow and ice areas of the Himalayas (Immerzeel et al., 2010), the rest coming from the rainfall within the basin, primarily during the monsoon season.

In the agricultural regions of the Ganges the average rainfall is between 600 and 800 mm, concentrated during the monsoon period, July to September. Outside the monsoon period and during monsoon breaks irrigation is necessary to maintain the high agricultural production. This leads to evaporation rates much higher than for rainfed agriculture (or natural forest cover).

In many studies the Ganges basin appears as a ‘hotspot’ particularly susceptible to land/atmosphere interactions. For example Koster et al. (2004) and Seneviratne et al. (2006) identify, within an ensemble of atmospheric models, some regions of high land-atmosphere coupling through the soil moisture, i.e. the Great Plains of north America, northern India and the Sahel. High evaporation rates associated with irrigated areas will lead locally to lower temperatures and higher humidity close to the surface, leading to reduced potential and actual evaporation (see e.g. Douglas et al., 2006). At a regional scale higher humidity in the lower atmospheric (or planetary) boundary layer may increase cloud cover and rainfall. The reduced temperature and surface turbulent heat flux may, however, reduce the depth of the planetary boundary layer and the generation of convection, and hence convective rainfall. The balance of these various effects will be determined by the magnitude of the surface fluxes and the stratification (temperature and humidity) of the lower atmosphere.

Classically the strength of the monsoon systems is regarded as a consequence of the land-sea temperature contrast however the situation is more complex, particularly for the South Asian monsoon which is a fully coupled ocean–land–atmosphere system (Turner and Annamalai, 2012). The land/sea warming contrast occurs over a significant depth of the troposphere and is initiated in the spring by surface sensible heating over the Tibetan Plateau, brought about by the increasing solar forcing, while latent heat release during the monsoon helps maintain the contrast as the solar forcing declines later in the year (Li and Yanai, 1996, Levine and Turner, 2012). Reduced surface temperatures due to irrigation would therefore be expected to reduce the intensity of the monsoon systems (Lee et al., 2009) however increased evaporation and latent heat released at height may ameliorate this effect. Douville et al. (2001) contrast the land surface influence in India and the Sahel and conclude that although precipitation does increase as a consequence of increasing evaporation this is counterbalanced, in the case of the Indian peninsula, by a reduced moisture convergence. Saeed et al. (2009) looked at these influences in more detail using a regional climate model, with and without irrigation. They found increased rainfall over the irrigated areas due to increased local moisture recycling and also an increase of the penetration of rain bearing depressions travelling inland from the Bay of Bengal, caused by a reduction in the westerly flows from the Arabian Sea. The overall impact of irrigation in the Indian sub-continent is complex and will depend on the thermodynamics of the atmosphere, the regional circulations and spatial scales of the irrigation. In paper we will discuss the strength of these different feedbacks from the extensive irrigated land and the meteorology at different scales: micro-scale (less than 10 km), meso-scale (or regional scale) (10 km to 100 km) and climate (or continental) scale (above 100 km) and their respective impacts on the water resources of the region (see Fig. 1).

Section snippets

Different drivers of evaporation in the Indian subcontinent

Evaporation at the land surface depends on the energy supply, the drying power of the air and water supply. The equation that most neatly summarises these controls on evaporation is the Penman-Monteith (Monteith, 1965) equation:λE=ΔA+ρcpδq/ra/Δ+γ1+rs/ra

Where λE is the latent heat of evaporation (W m 2), Δ is the rate of change of the saturated humidity with temperature (K 1), A is the available energy (W m 2), ρ is the density of the air (kg m 3), cp is the heat capacity of the air (J kg 1 K 1), δq

Planetary boundary layer (PBL) feedbacks

The planetary boundary layer is the part of the atmosphere that is impacted by the surface during the course of a day. During the day, the sun warms the surface and the surface warms and moistens this layer of the atmosphere through the turbulent fluxes of sensible heat and moisture. The second part of the feedback chain is the impact of evaporation (and also sensible heat fluxes) on the PBL moisture and temperature and the subsequent effect that has on evaporation itself. This straightforward

Monsoon system

The monsoon system directly controls the transport of water into the region and is an atmospheric response to the contrast between the heating of the land surface and the oceans. The land warms quickly in the spring and the ocean slowly, retaining its heat for longer. The subsequent impact on the dynamics of the global climate is that there is a strong seasonality to the weather in the Indian region.

The impact can be altered by the strength of the contrast between the land and the ocean.

Discussion and conclusions

Interactions between the atmosphere and the land surface, particularly irrigation, will occur on many scales. To a large extent the magnitude and form of these will depend on the depth of the atmosphere which is affected. Thus at the local, or micro, scale it is the surface layer some 10s m deep which is cooled and moistened by the evaporation of irrigated water reducing the potential evaporation. Calculations presented here show that over the Ganges region the evaporation demand is primarily

Conflict of interest

The Authors know of no conflicts of interest with respect to this paper.

Acknowledgements

A. Wiltshire was partly supported by the Joint DECC/Defra Met Office Hadley Centre Climate Programme (GA01101) and the HighNoon project funded by the European Commission Framework Programme 7 under Grant Nr. 227087. Obb Tuinenberg was also part funded by the EU HighNoon Project. Richard Harding and Eleanor Blyth were supported by the NERC Changing Water Cycle Programme and core funding.

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