Jan 25, 2023
UC Davis looks at vegetation for infiltration

Facing a fourth consecutive year of drought conditions across California, groundwater levels dropping throughout much of the Central Valley, and SGMA policies coming into effect, more and more California growers are looking to incorporate some form of agriculturally managed recharge (AG-MAR) into their water management plans. During the off-season for orchards, AG-MAR looks to divert runoff from winter storms into groundwater basins where the water can be “banked” for future use.

Since the primary factor governing the suitability of an orchard to AG-MAR is soil type, an online tool called the Soil Agricultural Groundwater Banking Index (SAGBI) was developed by the California Soil Resource Lab at University of California (UC), Davis. Using data including topography, percolation rates, and the root-zone residence time of applied water, the interface allows users to see suitability ratings for soils at most locations throughout the state.

The website features tabs where users can quickly toggle between unmodified ground conditions and a theoretical model that accounts for the typically improved conditions that result from deep tillage, having occurred, such as ripping prior to orchard establishment. Growers with operations in areas rated “moderately good” to “very poor” can still benefit from using vegetation to help increase infiltration, but need to take extra care to monitor soil moisture to avoid “wet feet” in their orchards, as the root-zone residence time of any water in the profile will be longer than in areas with higher SAGBI ratings.

Maps displaying data from the SAGBI website on soil suitability around the Orland, California, area. The map on the right displays theoretical data assuming significant improvement of AG-MAR suitability due to deep tillage.

Once the soil suitability is determined, establish or maintain vegetation in recharge areas. The presence of vegetation, whether resident vegetation or a cover crop, will help “slow the flow” of water over the surface and increase infiltration into the soil profile. Grasses are relatively deep-rooted and withstand water flows well, but any cover crop or resident vegetation will aid the capture and infiltration of stormwater. Grasses may also help suppress the growth and spread of undesirable weed species.

Legumes, such as clover, provide extra nitrogen to the soil, in addition to slowing water flows. They have a lower carbon-to-nitrogen ratio than grasses, providing a quicker decomposition if they are mowed or incorporated into the soil during the growing season. They’re a good choice for orchard middles, where grasses are likely better suited to channels, borders, and basins.

Brassicas (mustard, radish and peas) can be useful in situations where a quick stand is needed, but they become “woody” and are more challenging to reincorporate into the soil if left to grow to maturity. They are deep-rooted and very good for water infiltration, while being excellent forage for pollinators when in flower.

Of course, you may have many ever-changing priorities often conflicting with one another. Managing vegetation for infiltration is no different. Vegetation on orchard floors (two inches or taller) during the winter months shades the soil and makes passive frost protection less effective since soil is not warmed by the sun during the day.

– Curt Pierce, irrigation and water resources advisor, University of California Cooperative Extension

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