Drought and Irrigation Management

By Konrad Mathesius

Our everyday lives are peppered with reminders of the drought. Bumper stickers decrying the tunnels, I-5 billboards blaming congress for post-apocalyptic scenes of desiccated orchards, the giddy excitement over the radio correlated with major rain events, are all telltale symptoms of the pressure that California’s watersheds are facing, and that I spend too much time driving.

The surly truth is on the horizon. The Sustainable Groundwater Management Act has given local agencies a greater ability to curtail water use, in many cases ending the seemingly unbreakable conviction that the water that flows under a given piece of land belongs to the landowner. Any grower should be aware that water is gaining value. Depending on whom you ask, the price of water for agricultural use can hover between $700 and $2500 per acre-foot.

As a soil scientist, I work in orchards and vineyards. Almond growers have been demonized in the wake of the drought, prompting a smorgasbord of reactions ranging from educational radio soundbites to more billboards, but many have begun to dig deep in finding ways to master their irrigation.

The most basic principle is making sure that you’re not wasting it. Blake Sanden (Kern county UCCE farm advisor) explained at a recent young orchard workshop that tracking soil moisture at multiple depths would give growers an idea of where water is being used and where it’s being wasted. Knowing how deep your feeder roots are located as well as the soil types in question is critical to designing irrigation layouts. Essentially, putting moisture sensors at several depths, or using a neutron probe to measure at foot-by-foot increments, growers should be able to see where moisture levels stay constant and where they are being depleted by roots. Likewise, if you know that your rootzone extends to 3 or 4 feet, but you see moisture that increases with irrigation sets or otherwise stays constant (once at field capacity) at 5 or 6 feet, you are likely overwatering, risking unnecessary leaching.

The anecdote to illustrate Blake’s point on the importance of knowing your soils was the story of a grower who irrigated his orchards on a 48 hour set once every two weeks. The trees in the fine sandy loam on one end of the orchard were fantastic. Walking the line between insufficient and excessive, the trees thrived all the while avoiding problems with root rot. The other end completely collapsed. The soil under the struggling trees was a coarse sandy loam. With a dearth of clay particles to hold onto, water was essentially falling out the bottom and by the time the next irrigation was due, the trees were already under intense stress.

The relationships between soil and irrigation strategies are the same in vines. Soil maps are critical in their ability to provide key information on water holding capacity and when to irrigate. In the midst of the drought, this seems like something worth looking into. But it’s not entirely good enough. Finite differences in soil textures can lead to relatively different outcomes in terms of available water capacity. The process of monitoring your soil moisture status should thus be seen as something that is ongoing.

Most growers are probably better at reading their vineyard’s water status than anyone else standing above the soil line, but ET measurements aren’t nuanced, they’re numerical. By taking advantage of neutron probe readings or real-time capacitance probes, growers can quantify their resources and plan ahead accordingly. The old-school pressure bomb and other stress-monitoring devices are invaluable tools when better understanding what a moisture reading in the soil means for the plant above ground. Unfortunately, that’s only one half of the equation.

The undeniable irony that comes with increased water use efficiency is that groundwater salinity and boron/ chloride toxicity (particularly in stream or groundwater flow from the Coast Range) are becoming more of an issue. Not only are salt and mineral concentrations changing in the soil as irrigated water is transpired, but certain aquifers are becoming more saline or concentrated over time. Understanding how much water would be required to leach accumulated salts or potentially toxic materials in the soil profile requires more than a hunch that it’ll be a good El Niño year, it requires that growers know the moisture status of their soil going into winter and also compels growers to fully understand their water report.

Granted, any step toward better management is positive and growers throughout California cannot be classified into any single tier of water management, but if the drought is a harbinger of change, rather than an anomaly, the wine industry may be involuntarily compelled to evolve, albeit faster than some of my verdantly zealous neighbors. If nothing else, the economic argument for maintaining solid profit margins will compel growers to meet the challenges presented by ongoing drought conditions.