Land-use & the monsoon

The monsoon circulation is primarily driven by large scale pressure gradients, created by the thermal disparity between the ocean and the land (Clift and Plumb, 2008:17). The South Asian monsoon should therefore be classified as a coupled atmosphere-ocean-land phenomena, with the land as influential as the ocean (Yasunari, 2007). Douglas et al. (2006) note that most studies have focussed on the larger scale dynamics of the monsoon, and as such relatively few studies have investigated the potential effects of regional land-use. With 18% of the world's population on just 2.4% of the global land surface, and the projection that India will become the world's most populous country by 2050 followed closely by China, the pressures on the environment are intensifying and any effect upon the South Asian monsoon could be increasing (Ray, 2011).

Although much of India had experienced deforestation and been transformed into agricultural land by 1700, significant agricultural expansion into the north-west around the Himalayan foothills began in 1940. Further intensification coincided with the significant change in farming practices, including the increased use of groundwater irrigation, with the Green Revolution of the 1960s (Roy et al., 2007). Douglas et al. (2006) looked at the influence of agricultural irrigation on evapotranspiration (vapour flux) and surface radiative balance and concluded that these can affect the local convection and rainfall. Furthermore, it was suggested that vapour flux and rainfall changes can influence monsoon and global circulations through teleconnections, if the components cover a large enough area. 

Figure 1 - GA analysis showing strong and negative
trends in rainfall (Niyogi et al., 2010)

Niyogi et al. (2010) employed an Empirical Orthogonal Function (EOF) approach to explore climatic trends within the South Asian monsoon region. They found that there been significant increases in summer rainfall over east India and reductions over north and north-west India over the last five decades. Interestingly two north-western states most affected by an intensification of irrigated agriculture since the 1960s, Punjab and Haryana, have experienced significant reductions in surface sensible heat flux leading to a reduction in rainfall (Douglas et al., 2006). Niyogi et al. (2010) performed additional Genetic Algorithm (GA) analysis, the use of which ties spatial patterns to strong temporal trends, which confirmed the same result (Figure 1). This trend has been linked to an increase in soil moisture and vegetation changes, both of which can effect the radiation balance, causing regional surface cooling (Niyogi et al., 2010) along with changing regional convection (Douglas et al., 2006). 

The culprit for the increase in soil moisture is the increased use of groundwater irrigation; the area utilised for paddy cultivation tripled during 1960-1990, with much of it occurring in the premonsoon season (up to 4 weeks earlier than in the past) due to the diminishing reliance on monsoon rains to kick-start the growing season. Bringing the growing season forward increases the normalized differential vegetation index at a crucial stage of monsoon circulation evolution. Through increased albedo, the intensity of the negative pressure anomaly can reduce and thus weaken the pressure gradient, influencing rainfall amount and distribution (Niyogi et al., 2010). Other factors such as urbanization could also influence the precipitation, however Kaufmann et al. (2007) demonstrate this effect reduces rainfall, but not during the summer monsoon as the mechanism overwhelms local urban effects. Arguably it is the intensification of agriculture and the associated surface feedbacks where the evidence is most compelling. Indeed, Douglas et al. (2006) suggest that monsoonal rainfall variability is regulated by antecedent land surface conditions. 

Due to the complexities of the South Asian monsoon system and the relatively youthful nature of this area of study, it is difficult to definitively attribute the changes in the monsoon to land-use change, although there is compelling evidence of its influence. Arguably, it is the role of land-use changes in exacerbating monsoonal effects, as opposed to their effect of upon the monsoon itself, that are of greatest importance. With the population increasing rapidly and the country getting no bigger, the proportion of the population living in 'at risk' areas may increase, and as a result the extent of our influence could become evermore significant.