David Robinson

Research Interests and Goals:

Dryland Systems

Drylands account for about 41% of the Earth’s land surface, with about one third of the world’s human population living in them (~2 billion people). Of this group about 90% of the people live in developing countries. Poverty, poor nutrition, health and water shortage are endemic problems in many dryland areas. Water shortage in these regions is projected to increase further due to increasing population, climate change, and alteration of land surface cover. Increasing water stress leads to a range of social, economic and environmental impacts that are a challenge to sustainable development. Population growth increases pressure on land and water resources to produce food, feed and fiber. Agriculture is the world’s biggest consumer of water, mostly through irrigation used to replenish soil moisture. Increased irrigation in many parts of the world also comes hand in hand with problems such as soil salinization and land degradation. In many African countries more than 40% of the annually available fresh water resources are used in agriculture. This extraction places increased pressure on availability of water resources for non cropped ecosystems such as range lands and savannas. In drylands there is a fine balance between a functioning ecosystem and land 0degradation causing desertification. Finding solutions that allow for sustainable development, balancing food production and the maintenance of healthy ecosystems are critical.

Soil moisture is the major limiting factor in these regions that results from low rainfall and high evaporation. In these systems it is the major control on soil physical and biogeochemical processes in the critical zone, across the land surface, and maintaining the structure, function and diversity of these dryland ecosystems. In the case of physical properties it affects soil strength, making it harder for roots to penetrate in dry soils. Soil temperature decreases, and gas diffusion becomes more limiting, in increasingly wet soils. Albedo changes with soil moisture content, which has important consequences for the energy balance. Biogeochemistry is primarily controlled through microbial activity, responding to changes in water content. Microbial activity is greatest in soils at ‘field capacity’, following drainage by gravity, and reduces with saturation or dehydration. Nitrification follows a similar trend but denitrification occurs primarily as the soil saturates in response to oxygen becoming limiting to the microbes. As a result of these processes CO2 production increases as the soil becomes moist and tends to plateau toward saturation.

In many areas soil fertility and soil salinity are constraints even when water is not. Soils are electrically charged and in temperate climates it is the predominantly negative charge that retains base cations such as potassium and calcium. In highly weathered tropical soils, dominated by iron and aluminium oxides, soils are variably charged with the pH determining the net negative of positive charge. Many soils tend towards the point of zero net charge, making it hard for these soils to retain plant nutrients; infertility becomes a major constraint. Soil salinity occurs with a build up of soluble salts in the soil profile, in many parts of the world it’s nick named the ‘White Death’. Salinity kills plants, reducing ground cover and making land more susceptible to erosion. Soil sodicity causes soil clays to deflocculate, leading to pore clogging and soil compaction. Reduced infiltration results in increased surface runoff and higher risk of erosion and decreased recharge.

My research focuses on dryland systems, integrating soil science, hydrology, ecology and geophysics. The main aspect emphasizes soil moisture, its measurement, spatial distribution and ecohydrological role. Other aspects include the determination of subsurface properties and structure across the landscape including, identification of flow paths, soil porosity and texture, and soil salinity. Much of the research has focused on the development of reliable soil moisture sensors, which are critical for use in dryland systems where small changes in soil moisture can have significant ecohydrological impact. Current research thrusts include: