1

Cooperative Extension, University of California

Water & Land Resource Manager TEHAMA, GLENN, COLUSA, AND SHASTA COUNTIES 1754 WALNUT ST, RED BLUFF, CA 96080 (530)-527-3101 Email: aefulton@ucdavis.edu February 2011

IN THIS ISSUEIN THIS ISSUE

• How Effective Have the 2010/11 Winter Rains Been So Far?

• 2010 Fall Groundwater Levels in Tehama County.

Allan Fulton UCCE Farm Advisor - Tehama, Glenn, Colusa, and Shasta Counties

HOW EFFECTIVE HAVE THE 2010/11 WINTER RAINS BEEN SO FAR? Allan Fulton and Richard Buchner

Winter rainfall and soil moisture storage is important because it directly affects irrigation manage-ment for orchard crops. Rain water replenishes soil moisture that was depleted in the previous sea-son. It occurs during the cool, winter season when trees are dormant and evaporative demand is low providing more opportunity for the rain water to infiltrate and percolate deeper into the soil profile where it is stored for plant transpiration after leafing. Deep soil moisture reserves are especially cru-cial in supplying tree water demands in July, August, and September when 1) crop water consump-tion is high, 2) when irrigation may be interrupted for harvest, and 3) when some soils have the slow-est water infiltration characteristics reducing water penetration. Sufficient winter rainfall ultimately reduces the amount of irrigation needed and related irrigation costs. The amount of stored soil moisture replenished in the tree root zone will depend upon soil type and tree rooting depth.

The 2010/11 fall and winter rainfall season in the northern Sacramento Valley started out relatively strong, from mid October 2010 through mid January 2011, rainfall totaled about 9.0 inches at the Gerber CIMIS weather station #8 and about 11.4 inches at the Durham CIMIS weather station #12. Since mid January and if the forecast holds true up to February 15, 2011, we will have received less than 0.5 inches of rainfall in the past four weeks plus unusually warm temperatures have lead to higher than normal levels of evaporative demand. Orchard managers need to know how effective the 2010/11 winter rains have been at replenishing the soil moisture.

2

Table 1 (page 3), provides a comparison of soil moisture levels measured in eight different orchard and soil types. Herbicides were applied to control weeds in the tree rows and resident vegetation was allowed to grow in the row centers. Soil moisture levels were measured in the tree row with a field calibrated, neutron moisture meter. For comparison, soil moisture levels are shown for March 3, 2010 at the beginning of the growing sea-son for each soil type. Almonds had bloomed and were in early canopy development and had consumed very little soil moisture. French prune was showing bud swell but had no leaves, and later leafing walnut va-rieties were still dormant. More rainfall had occurred in the 2009/10 winter season than in the 2010/11 sea-son. About 17.3 inches of winter rainfall had been recorded at the Gerber CIMIS station and 12.9 inches at the Durham CIMIS weather stations. These soil moisture levels represent conditions described as “field ca-pacity. When soil moisture is at field capacity, it has high moisture content but any excess water has drained so the soils are not saturated. Soils at field capacity have the maximum plant available water and are well aerated to support healthy root development.

For comparison, soil moisture levels are also shown for October 20, 2010 for each soil type. These levels represent the soil moisture conditions near the end of the 2010 irrigation season towards the end of harvest and after varying degrees of post-harvest irrigation and fall rains. The difference between the soil moisture levels on March 3, 2010 and levels on October 20, 2010 shows varying levels in depletion of the plant avail-able soil moisture in each of these soil types. It also provides a measure of the replenishment needed to be-gin the 2011 irrigation season at field capacity and with a full profile of stored soil moisture in each of these soil types.

The soil moisture levels reported for February 8, 2011 represent relatively current soil moisture conditions in these soil types. It shows how effectively rainfall that has been received up to this point in 2010/11 winter sea-son has been at replenishing stored soil moisture in orchards.

Table 1 shows the increase in soil moisture in the six foot profile from October 20, 2010 through February 8, 2011 varied widely across all eight soil types. The lowest was 1.42 inches on a Hillgate loam soil where ma-ture French prune was irrigated with single line drip irrigation (Orchard #5) and the highest was 7.66 inches for the Vina loam soil where a 6th leaf walnut orchard (#7) was irrigated with rotator minisprinklers. This repre-sents a range from 15 to 85 percent effective storage of rainfall in these soils. Many factors that reflect site specific differences in crops, soils, weather, water table conditions in proximity to the Sacramento River, irri-gation systems, and management influence how effectively the winter rainfall has been stored. For instance, the soil moisture depletion from March 3, 2010 through October 20, 2010 was much higher, 10.64 inches, in the 6th leaf walnut orchard (#7) on the Vina loam soil and only 2.0 inches for the French prune orchard (#5) grown on the Hillgate loam soil. The capacity of the Vina loam soil to store rainfall was much higher and the drier Vina loam soil was likely to have higher water infiltration and percolation characteristics.

Table 1 also shows that some of the orchard soils have been replenished to field capacity within the six foot profile by the winter rains while others have not yet been fully refilled. The mature walnuts grown on the Far-well loam soil (Orchard #3) and Gianella fine sandy loam soil (Orchard #4), the French prunes on the Hillgate loam soil (Orchard #5), and the mature almonds on the Tehama silt loam soil (Orchard #6) are examples where the soil moisture levels on February 8, 2011 in the six foot profile were at or near field capacity and closely matched the previous spring levels of soil moisture (March 3, 2010) when total rainfall had been higher. The 4th and 5th leaf walnuts grown on the Arbuckle gravelly loam (Orchard #1), the mature walnuts grown on the Cortina gravelly fine sandy loam (Orchard #2), the 6th leaf walnuts on the Vina loam (Orchard #7), and the mature walnuts on the Wyo silt loam (Orchard #8) are examples where soil moisture in the six foot soil profile has not yet been fully replenished to field capacity. An additional 1.2 to 4.0 inches of, effective rainfall is needed to refill the six foot soil profile in these orchard soils.

To summarize, the 2010/11 rainfall season started strong but has faded in the last month. Soil moisture has been replenished to field capacity by rainfall in some orchard soils to a depth of six feet but not in others. As much as about 4.0 inches of additional, effective rainfall in February and March may be needed to replenish deep soil moisture in some orchard soils. Many factors may contribute to a wide range in soil moisture re-plenishment from one orchard to the next. Taking the time to auger and visually evaluate or investing in soil moisture monitoring methods can help assess site specific conditions. Be particularly attentive to orchards where vigorously growing cover crops are currently extracting soil moisture prior to the orchard leafing and plan irrigation to avoid early season tree stress and to favor moisture storage for use later in the season.

3

4

2010 Fall Groundwater Levels in Tehama County Allan Fulton

Land and water use analysis by the California Department and Water Resources Northern Region in 2005 showed that Tehama County acquires 69 percent of its total, annual water demand from groundwater. Reliance on groundwater is significantly higher in Tehama County than in the neighboring counties of Shasta, Glenn, Butte, and Colusa where groundwater provides between 16 to 31 percent of the total, annual water demand. Sur-face water resources supply a much larger fraction of the total water demand in Shasta, Glenn, Butte, and Colusa counties than Tehama County.

The 2003 Water Inventory Analysis for Tehama County provides more insight to Tehama County’s reliance on groundwater. The larger surface water supplies in Tehama County are from tributaries on the eastside of the Sacramento River. The Central Valley Project (Shasta Reservoir) provides a smaller proportion of the surface water supplies. Concerns about reliability and flexibility of surface water resources and other factors such as contin-ued shifts to orchard crops and drip and microspinkler irrigation have also contributed to an increasing reliance on groundwater in Tehama County over the last two decades.

Recognizing its reliance on groundwater and after a great deal of public input, the Tehama County Flood Control and Water Conservation District adopted a Coordinated (AB 3030) Groundwater Management Plan in November 1996 and has been working to implement it since then. One of the main purposes of the Plan has been to “develop a comprehensive groundwater basin management program which protects the groundwater resources of Tehama County in order to provide local users with a reliable long-term water supply”. Over the years, routine groundwater level monitoring and analysis of the groundwater level data has become an important part of the Plan to achieve this purpose.

Spring, summer, and fall groundwater levels have been measured annually at about 190 monitoring locations in Tehama County. A combination of irrigation, domestic, and dedi-cated groundwater monitoring wells are used to measure and track groundwater levels. In 2009, a subset of these monitoring locations was selected as “Key Wells” and in following the Groundwater Management Plan “Trigger Levels” and “Awareness Actions” were de-fined for these key wells. Several criteria were considered when the selecting the Key Wells such as 1) broad geographic distribution throughout the 12 groundwater sub-basins in the County, 2) long history (20 or more years) of groundwater level monitoring that pro-vide baseline trends and long-term variation, 3) known well construction features and good representation of other surrounding wells, and 4) landowner permission to access monitor-ing sites and take measurements.

5

Following the Groundwater Management Plan, three seasonal Trigger Levels were established in 2009, a “Spring Trigger Level 1 and 2” and a “Late Season Trigger Level”. Trigger Levels are established specifically for each Key Well. The Spring Trigger Level 1 is calculated as the historical low of the spring measurements plus 20 percent of the range in historic spring groundwater levels. The Spring Trigger Level 2 is defined as the historical low groundwater level in the past 20 to 40 years and typically corresponds with one of the more sever periods of drought that occurred in 1977 or 1991. Spring groundwater levels are important because they provide an indicator of how sufficiently the groundwater levels recover each year after the rains, snowpack, and runoff season and before the more intensive season of groundwater extraction begins. The Late Season Trigger Level represents the historical low groundwater level meas-ured during late summer to early fall (August – October). The Late Season Trigger Level also usually corresponds with groundwater levels measured during the 1977 or 1991 drought. The Late Season Trigger Level provides an indicator of groundwater conditions near the end of the more intensive season of groundwater extraction.

Comparing routine measurements of groundwater levels to these trigger levels provides a sys-tem to signal when groundwater levels are declining to levels that have not been experienced in the past. When monitoring indicates that groundwater levels have declined below a Trigger Level, the Groundwater Management Plan calls for attention to the matter. A series of “Awareness Actions” have been defined as response steps. The Awareness Actions focus on communication, education, investigation, and cooperation to manage the groundwater resource

Table 1, on pages 6 and 7, summarizes groundwater levels measured in August 2009, March 2010, August 2010, and most recently in October 2010 in 9 of 12 groundwater sub-basins of Tehama County. These groundwater levels are also compared to the Spring and Late Season Trigger Levels. The monitoring shows a general pattern of continued recovery of groundwater levels from the drought conditions that occurred from 2007-2009. Among the most recent groundwater level measurements taken in October 2010, two of the 35 Key Wells showed groundwater levels that surpassed Trigger Levels. One Key Well located in the Corning East Sub-basin was 3.7 feet lower than the Late Season Trigger Level and the groundwater level in one Key Well in the Vina Sub-basin was 2.1 feet lower than the Late Season Trigger Level. The Tehama County Flood Control and Water Conservation District and the AB 3030 Ground-water Management Plan Technical Advisory Committee are currently gathering more informa-tion to better understand the situation surrounding these two key wells.

The next round of groundwater level measurements will be completed towards the end of March 2011. Refer to the website http://www.tehamacountywater.ca.gov/ for more information on groundwater management efforts in Tehama County.

6

7

8

Cooperative Extension, University of California

Water & Land Resource Manager Newsletter TEHAMA, GLENN, COLUSA, AND SHASTA COUNTIES

___________________________

Allan Fulton

UC Farm Advisor

The University of California prohibits discrimination or harassment of any person on the basis of race, color, national origin, religion, sex, gender idenity, pregnancy (including childbirth and medical conditions related to pregnancy or childbirth), physical or mental disability, medical condition (cancer-related or genetic characteristics), ancestry, martial status, age, sexual orientation, citizenship, or status as a covered veteran (covered veterans are special disabled or any other veterans who served on active duty during a war or in a campaign or expedition for which a campaign badge has been authorized) in any of its programs or activities. University policy is intended to be consistent with the provisions of applicable State and Federal laws. Inquiries regarding the University’s nondiscrimination policies may be directed to the Affirmative Action/Staff Personnel Services Director, University of California, Agriculture and Natural Resources, 300 Lakeside Drive, 6th Floor, Oakland, CA 94612-3550. (510) 987-0096. To simplify information, trade names of products may have been used but no endorsement of named product is intended, nor is criticism implied of similar products, which are not mentioned.

Cooperative Extension Work in Agriculture and Home Economics, U.S. Department of Agriculture, University of California, and County of Tehama Cooperating.

Cooperative Extension PRSRT STD University of California US POSTAGE 1754Walnut Street Paid Red Bluff, CA 96080 Red Bluff “52” Permit No. 112