The Soil Agricultural Groundwater Banking Index (SAGBI) is a suitability index for groundwater recharge on agricultural land. The SAGBI is based on five major factors that are critical to successful agricultural groundwater banking: deep percolation, root zone residence time, topography, chemical limitations, and soil surface condition. More details can be found in the SAGBI article in California Agriculture.
The SAGBI is based on the following factors:
For successful groundwater recharge, soils must be able to transmit water into the aquifer below. The deep percolation factor is derived from the soil horizon with the lowest saturated hydraulic conductivity, or Ksat. Saturated hydraulic conductivity is a measure of soil permeability when soil is saturated.
Prolonged duration of saturated or nearly saturated conditions in the root zone can cause damage to perennial crops. The root zone residence time factor estimates the likelihood of maintaining good drainage within the root zone shortly after water is applied. This rating is based on the harmonic mean of the saturated hydraulic conductivity (Ksat) of all horizons in the soil profile, soil drainage class and shrink-swell properties.
Salinity is a threat to the sustainability of agriculture and groundwater in California. The chemical limitations factor is quantified using the electrical conductivity (EC) of the soil, which is a measure of soil salinity.
Agricultural groundwater banking will likely be implemented by spreading water across fields. Level topography is better suited for holding water on the landscape, thereby allowing for infiltration across large areas, reducing ponding and minimizing erosion by runoff. Ranges in slope percent are used to categorize soils into five slope classes: optimal, good, moderate, challenging, and extremely challenging.
Groundwater banking by flood spreading can subject the soil surface to changes in its physical condition. Depending on the water quality and depth, standing water can lead to the destruction of aggregates, the formation of physical soil crusts, and compaction, all of which limit infiltration. Two soil properties are used to diagnose surface condition: sodium adsorption ratio (SAR) is used to identify soils prone to crusting, and the soil erosion factor (Kw) is used to estimate the potential soil susceptibility to erosion, disaggregation, and physical crust formation.
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|SAGBI Rating||Rating Class|
|85 - 100||Excellent|
|69 - 85||Good|
|49 - 69||Moderately Good|
|29 - 49||Moderately Poor|
|15 - 29||Poor|
|0 - 15||Very Poor|