Relating Soil Organic Carbon Increases to Available Water Storage and Drought Vulnerability in South-Central Idaho
Abstract
Preventing topsoil loss and increasing soil water holding capacity is critical to the sustainability of dryland, drought-prone agricultural areas. The growing pressure on freshwater resources combined with an increased likelihood of drought requires that scientists, policymakers, and land managers alike consider all mechanisms to ensure food and water security and ecosystem integrity. The benefits of conservation agricultural practices on soil structure, fertility, and water holding capacity are well-known. Land managers, however, are often slow to adopt them for fear of financial losses and other diseconomies.This thesis examines the impacts of building soil organic carbon (SOC) stocks from conservation agricultural practices on available water storage, drought vulnerability, and aquifer recharge in two agricultural areas of south-central Idaho: The Eastern Snake Plain and the Wood River Valley. Baseline SOC and available water storage (AWS) values were taken from gSSURGO, the U.S. Department of Agriculture’s national soils database. To estimate SOC increases, I applied carbon (C) sequestration values derived from the scientific literature to four agricultural land types (alfalfa, barley, pasture, and shrub land). I then calculated the impact of increasing SOC using four parameters: 1) the time needed to reverse historic SOC losses from tillage; 2) the quantity of AWS increases over ten years; 3) the time needed to reverse “drought vulnerability” (< 0.5 af of AWS in the rootzone); and 4) the reductions in aquifer recharge from increased AWS. I analyzed the on-farm financial implications of adopting no till practices and compared the cost-efficacy of these practices for soil-building to expenditures under the federal Conservation Reserve Enhancement Program (CREP).
The major conclusions from this work are:
1. Conservation agricultural practices can reduce historic SOC losses from tillage in relatively short periods of time (often one year or less).
2. These practices can also significantly increase soil AWS and thus may contribute to both water conservation and resiliency during drought and/or water shortages.
3. For shrub and pasture land, rotational grazing and other conservation agricultural practices can reverse drought vulnerability in relatively short periods of time (five to seven years), and these results imply even faster rates for other types of cropland.
4. While significant gains are likely in terms of resiliency, conservation agricultural practices can significantly impact recharge rates and therefore should be considered both in hydrological modeling and policy development.
5. While small farms in Idaho may incur a cost in moving to no till, large farms may gain financially, an important consideration given that farm sizes are increasing.
6. As compared to the CREP, no till is likely more cost effective for soil building.
This thesis concludes with a set of interrelated research and policy recommendations. Chief among these are the need to more directly establish the relationship between increasing AWS and net irrigation requirements and to employ a strategic, integrated impact assessment process to ensure that policies support a sustainable future by protecting water and land resources.
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