Sustainable water management is one of the most challenging issues of our time, especially in the arid western U.S. Adequate water supplies are crucial to maintaining the health of communities, rivers, and wildlife, and nothing is more important to agriculture's ability to produce food for the world's growing population. Maximizing the benefits of our water supplies requires careful measurement of their availability and use. For irrigated agriculture, satellite-based estimates of evapotranspiration (ET) provide a measure of the water used to grow food — the biggest share of water consumption in most arid environments around the world. However, access to this data has been limited and expensive, keeping it out of the hands of most water users and decision-makers.

OpenET provides open, easily accessible satellite-based ET data for improved water management. The OpenET collaborative includes leading national and international experts in remote sensing of ET, cloud computing, and water policy, partnered with nationally recognized web development teams and leaders in the western agriculture and water management communities.

This data site can focus in on a field or orchard using Google Earth and provide a monthly and yearly water use for that orchard. It's not the potential water use – the amount of water that could be used in the field driven by weather elements – but the actual water use, the consumptive use. If the field is under irrigated, it would show how little water is being lost there. If the field is over irrigated, it will show the potential ET.

At the moment, the data is not for daily use, although that is forecast to occur later in the year. It also has somewhat clunky navigation, but with patience, it's possible to get around to finding an individual field.  It's a work in progress, but it could be a great tool soon.

It's free and you can check it out at: https://openetdata.org/about/

Efficient and precise irrigation management is critical if California producers are to maximize crop quality, conserve water, and protect the environment. The use of evapotranspiration (ET) estimates is a significant component of irrigation management. ET refers to the sum of water lost from the soil (evaporation) as well as that used by the crop (transpiration). While the California Irrigation Management Information System (CIMIS) network of weather stations derive daily ET values, there is a perception that CIMIS does not produce accurate ET estimates for all locations. This view is particularly prevalent in the canyons of Ventura County where weather conditions differ substantially compared to CIMIS locations. Since avocado and citrus thrive in these areas, it was concerning when it was determined that ET scheduling is not widely used.

That is, a Ventura County Resource Conservation District (RCD) review of California Department of Food and Agricultural State Water Efficiency and Enhancement Program (CDFA SWEEP) projects concluded that Ventura County growers substantially lagged their state-wide peers with respect to implementing ET-based irrigation scheduling (14% versus 44%).

RCD seeks to reverse the low implementation of ET-based irrigation scheduling within Ventura County by using simple, rugged on-site ET devices (atmometers) to determine on-site ET values. These on-site values will be compared to CIMIS values to determine local correction factors and develop refined ET maps for the canyon and valley areas. RCD will present these results at outreach events and provide workshops demonstrating how ET data, whether from CIMIS or on-site atmometers, can be used for irrigation management.

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PHOTO: Atmometer Test/Calibration Site @ UC Hansen

Many orchards in California are planted on slopes, the most extreme examples are usually avocado orchards with some slopes exceeding 50%. They pose difficulties in harvesting because of the steepness, but also in their irrigation. These slopes can be north/south/east/west facing or all of the quadrants in the same orchard. The plantings can be of varying steepness and at different positions (toe, top, mid-slope). These positions affect solar radiation which is the main driver of evapotranspiration, but also wind interception. South and west facing slopes intercept the most sunlight, while north and east intercept the least. The top of the slope usually intercepts the most sunlight during the day and also the most wind. There can be 100% difference in the amount of ET depending on the position on the slope. That is, some trees require twice as much water as others because they are getting more energy that drives water loss.

When looking at an older avocado grove, the trees are usually larger at the bottom of the slope where there is the least wind and most irrigation water interception. This is where the soil is the deepest and has the greatest moisture reserve. The soils at the top of the slope are the shallowest and get the greatest amount of energy driving water loss. Trees on the north side are often tall from greater soil depth and moisture reserve and less ET demand. As the solar angle changes during the year (lower in the sky during the winter), the proportion of ET in these different positions changes.

Right. OK. We know this. The problem is that many smaller orchards are laid out so that there is one valve controlling the amount of water going to all the different positions. The trees at the bottom of the slopes get the same as those at the top. Those on the north side get the same as those on the south side. This basically sets up an orchard for stress. Stress that leads to disease and impacts on yields and ultimately the longevity of the orchard.

Add to this, irrigation performance varies with pressure and many orchards have very little pressure compensation. Often trees at the top of the slope have the lowest pressure and output. The distribution uniformity is often terrible. Not only are the normal problems of broken and clogged emitters an issue, but also pressure loss from elevation differences.

So where you plant on a hillside should be part of the irrigation design. In different positions on the hillside there are different water requirements and unless they are irrigated differently, there can be major differences in tree response. These different irrigation requirements should be incorporated into the irrigation design by creating as many different irrigation blocks as possible. A valve for the top of the slope, another for the north and south slopes, etc. These can be incorporated easily in the initial design and not so easily customized after the trees have been planted.

At some point, for optimum tree performance, tree health and water use efficiency, growers should recognize the need for irrigating to trees' needs according to slope position. Avocado growers have it harder than most growers.

Read more about an ET study done on a hill:

http://www.avocadosource.com/CAS_Yearbooks/CAS_86_2002/cas_2002_pg_099-104.pdf

Capturing Precipitation - How much rainfall do I need to capture?

Managing precipitation to your advantage is really a three step process (Lal and Stewart, 2012).

ü  Step 1 - maximize preciptitation captured in the soil

ü  Step 2 - minimize the evaporation of the stored soil moisture

ü  Step 3 - maximize plant water use efficiency

The first step of the process is often thought of as “effective rain”. Effective rainfall refers to the percentage of rainfall which becomes available to plants and crops. It considers “losses” due to runoff, evaporation and deep percolation (Klein, 2011).  In the past we might have considered deep percolation as a loss. We now know that percolation “losses” may be a vital resource in sustaining our groundwater basins. As we move into the fall of 2015, we have the opportunity to plan for effective rainfall by managing the orchard floor for maximum capture of precipitation. This will help provide stored soil moisture for plant growth as well as deep percolation of water to groundwater

The following figure illustrates some of the important points about effective rainfall and reminds us of what we can do to maximize capture of precipitation (1). We want to maximize 2 (infiltration during a rain event), 3 (surface capture), 6 (infiltration from surface capture), 7 (percolation to ground water), and 8 (rootzone storage for use by the crop). We want to minimize 4 (runoff) and 5 (evaporation).

When rain water ((1) falls on the soil surface, some of it infiltrates into the soil (2), some stagnates on the surface (3), while some flows over the surface as runoff (4). When the rainfall stops, some of the water stagnating on the surface (3) evaporates to the atmosphere (5), while the rest slowly infiltrates into the soil (6). From all the water that infiltrates into the soil ((2) and (6)), some percolates below the rootzone (7), while the rest remains stored in the rootzone (8). From FAO Irrigation Water Management 1985 http://www.fao.org/docrep/r4082e/r4082e05.htm#4.1.4 effective rainfall

Larry Stein from Texas A&M wrote a very good basic explanation “So What Constitutes an Effective Rain Event ?” (Stein, 2011) We can use his approach to look at managing precipitation in the Central Coast. Understanding these concepts can help you manage precipitation in your operation.

For example, the majority of olive roots are in the top 18 inches of soil. So how much rainfall do we need to capture to refill the rootzone of an olive grove in Paso Robles? We need to know:

ü  The amount and intensity of rainfall

ü  The infiltration rate of the soil (how fast the soil takes in water). Sandy soils take water in more quickly.

ü  How much water the soil will hold in the rootzone of the grove

Average rainfall for Paso Robles in January is about 2.75 inches. Table 1 shows that olives on a sandy loam soil might be able to infiltrate 1 to 1.5 inches per hour. If all that rain comes in one storm then as much as 1.25 inches may either run off (4) or pond (3) in the low spots until it can infiltrate.

Average rainfall in Paso Robles in January would be adequate to refill the rootzone of olives (8) on a sandy loam soil, IF all of the rainfall infiltrates (2), and none is lost to evaporation (5) or runoff (4).

Table 1. General soil water storage and depletion characteristics for three different soil types (Klein, 2011)

Cover crops help keep the soil surface from crusting as well as protecting the soil surface from erosion. Their roots provide channels for water to infiltrate into the soil. Remember that cover crops may also be using water stored in the rootzone (8). When facing drought conditions, it may be advantageous to manage with low residue cover crops to reduce the amount of water extracted from the rootzone.  Here's a link to a video on low residue cover crops and their impact on runoff from work by UC Cooperative Extension Advisors in Monterey County https://www.youtube.com/watch?v=k0oVVJ_BA7s

Klein, L. 2011. So What Constitutes an Effective Rain Event? http://aggie-horticulture.tamu.edu/earthkind/drought/drought-management-for-commercial-horticulture/so-what-constitutes-an-effective-rain-event/ .

Lal. R.and, ‎B.A. Stewart. 2012. Soil Water and Agronomic Productivity