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Abstract:
In the coming decades, one of humankind’s main challenges will be to guarantee
food supply for a growing population while managing ever more scarce resources,
and safeguarding the availability of water and land for natural ecosystems. Brazil is
a country with abundance of water and land resources, and has recently become one
of the world’s main exporters of agricultural commodities. The Brazilian agricultural
sector recently went through unprecedented intensification and extensification
processes, which include the expansion of cropland and pasture areas into vulnerable
ecosystems.
This PhD thesis advances the research field on water and land resources assessment
by investigating the following processes: i) changes in green water use by
major rainfed crops in Brazil during recent decades ii) potential improvements of
land and green water productivity for Brazilian rainfed crops, with and without
implementation of supplemental irrigation, iii) influences of intensification and
expansion of soybean production in Brazil on green water use, and iv) impacts of
double-cropping on water use intensity.
The process-based biogeophysical crop model EPIC was chosen and applied to
simulate water use and crop growth for Brazilian rainfed crops, under diverse
management conditions. In Chapter 2, I simulated yields, water use and water
productivity for soybeans, maize, cotton and wheat under different scenarios of
agricultural management, for rainfed and irrigated conditions. Chapter 3 focuses on
soybean and maize, and analyzes water use for the production of these crops under
single and double-cropping conditions.
The results show an increase in green water use for the production of maize,
cotton, soybeans and wheat from 145 to 263 km3 per year between 1990 and 2013.
This increase is in large part due to the expansion of harvested area for soybean
production. The analysis of management scenarios shows that improving nutrient
management has a larger potential to improve land and water productivity compared
to supplemental irrigation. Furthermore, supplemental irrigation would lead to a
reduction in water productivity despite marginal improvements in land productivity.
The analysis of double-cropping practices for soybean and maize production systems
finds a greater water appropriation of these systems avoiding cropland expansion
and improving overall water productivity.
This thesis improves the understanding of current and future use of land and
water, the role of intensification and expansion processes, and interactions between
land and water use. The results lead to the main conclusion that harvested area
expansion has been a major driving force in the increase of green water appropriation
in Brazil in the last decades, but that different intensification processes have led to a more productive use of water and land over time. More importantly, the growth of
double-cropping has allowed a decoupling between the appropriation of water and
land, allowing a great increase of agricultural water use without further expansion of cropland.
