How Does Soil Carbon Farming Increase the Effective Rainfall on a Farm?
Soil carbon farming plays a crucial role in enhancing a farm's effective rainfall, particularly in the context of a changing climate. In the Southwest Land Division of Western Australia, average rainfall has significantly declined, posing severe challenges for farmers.
According to the September 2021 Western Australian Climate Projections report by the Western Australian Government (which references projections from CSIRO and the Bureau of Meteorology), the region has experienced a marked drying trend since 1970. This trend is expected to continue, with forecasts indicating further decreases in rainfall and increased drought conditions. Soil carbon farming can mitigate these challenges by improving soil structure and its capacity to retain water.
Effective rainfall, also known as useful rainfall, refers to the portion of total rainfall that is actually available for crop use. Not all rainfall contributes to plant growth; effective rainfall is the amount that infiltrates the soil and is accessible to plants, supporting their growth and development. This concept is crucial in agriculture, as it helps farmers determine water needs for their crops and livestock.
Challenges for Farmers
Farmers in the Southwest Land Division face several challenges due to climate change and salinity:
Unpredictable Rainfall: Less reliable rainfall patterns make it difficult to plan planting and harvesting schedules;
Water Scarcity: Reduced rainfall increases competition for water resources, making efficient water management essential;
Soil Degradation and Salinity: Intensive farming practices, climate change, and salinity can lead to soil degradation, reducing its ability to retain water and support crops; and
Increased Drought: Under all climate change scenarios, the time spent in drought is projected to increase, exacerbating water stress on crops and soils.
Climate Change and Reduced Rainfall
The Southwest Land Division has experienced a significant decline in rainfall, particularly in autumn and early winter, with a decrease more substantial than anywhere else in Australia. Projections indicate that by 2030, winter rainfall may decrease by up to 15% under low, medium and high emission scenarios. This reduction in total annual rainfall not only poses a challenge for farmers but also intensifies the heavy rainfall events, further complicating water management for them.
Enhanced Soil Structure
Soil carbon farming involves supporting the life of the soil, to provide fertility to the crop, naturally control pests and disease, and maintain the structure of the soil to capture rainfall and store soil moisture effectively. With the variety of modern technology available to farmers today, there are many interventions that a farmer can apply to achieve the above outcomes.
Increased Water Retention
Maintaining a living root in the soil and maximising crop residue retention over summer lowers soil temperatures, mitigates against wind erosion, reduces evaporation, and supports soil biological mineral cycling. Soil porosity, aeration, and aggregation are maintained, supporting rainfall infiltration and water retention after a rainfall event. This stored water is then available for plants over extended periods, particularly during dry spells. This increased water retention is critical for sustaining crops and pastures in regions facing reduced rainfall.
Reduced Erosion and Runoff
Improved soil structure and increased soil organic matter content reduce soil erosion and surface runoff. When rainwater infiltrates the soil more effectively, less water is lost to runoff, and more is retained within the soil.
If we were able to increase soil carbon by around 0.01% each year, this would mean that an extra 870L of water per ha/yr would be retained with each 1mm of rain. For a farm in the Wheatbelt of WA, which receives, for example, 300mm of rain over a 6-month winter growing season, this equates to an extra 26.1 mm over the growing season. (Using the calculation 870L multiplied by 300 gives you 261,000L, which converts to 26.1mm using the conversion that 1 mm rain equals 10,000 litres per hectare). Source: Bagnall et al (2022).
Combating Salinity
Salinity is a significant issue for farmers in the Southwest Land Division. Soil carbon farming can help mitigate salinity by improving soil health and increasing soil organic matter content. Healthier soils with better structure and water retention capabilities are less prone to salt accumulation. Furthermore, deep-rooted plants and cover crops used in carbon farming can help lower the water table, reducing the rise of saline groundwater.
Improved Plant Health and Yield
Carbon-rich soils, resulting from soil carbon farming, support healthier plant growth by providing essential nutrients and maintaining optimal moisture levels. This leads to plants with robust, deep root systems that can access water stored deeper in the soil profile, making them more resilient to periods of low rainfall. The result? Even under challenging climatic conditions, better crop yields and more consistent agricultural productivity.
Conclusion
Soil carbon farming offers a sustainable solution to the challenges posed by climate change and salinity, particularly the declining rainfall in the Southwest Land Division of Western Australia. By improving soil structure and increasing water retention, soil carbon farming helps farmers make the most of available rainfall, ensuring that their crops have a reliable water supply and enhancing the resilience of their farming systems.
