|Ayars, James - Jim|
Submitted to: California Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/22/2004
Publication Date: 5/1/2005
Citation: Johnson, R.S., Williams, L.E., Ayars, J.E., Trout, T.J. 2005. Weighing lysimeters for studying tree and vine water relations.. California Agriculture. Apr-Jun 2005, pgs. 133-136. Interpretive Summary: Efficient water use is critical is water-short areas such as the western US. Weighing lysimeters are a precise method to determine the water requirements of plants. The University of California and USDA-ARS-Water Management Research Unit, Fresno, have cooperated for nearly 20 years on water use studies for peaches and grapes using jointly installed and maintained weighing lysimeters. This is a unique facility world wide and has allowed determination of precise crop coefficients for these crops. With these coefficients, growers around the world can accurately predict water needs and schedule irrigations for peach and grape crops.
Technical Abstract: In 1986, two large weighing lysimeters, one in a grape vineyard and the other in a peach orchard, were constructed at the Kearney Agricultural Center as a joint effort between UC and the USDA Water Management Lab. Hourly weight changes in the lysimeters have been used to measure daily and seasonal water use of trees and vines. Peach and grape exhibit similar seasonal crop coefficient (Kc = ETc/ETo) patterns that start as low as 0.1 in March, increase linearly until early July and then remain constant between 1.1 and 1.2 for the remainder of the season, provided the plants remain healthy (disease, pest and stress free). The linear increase phase is proportional to the increase in leaf area and canopy light interception. These relationships have led to modeling efforts to predict ETc for varying conditions. The peach lysimeter has also been very useful for studying the effects of water stress on tree water use and for comparing other methods of estimating water use. There is extensive literature on water use by plants. Many factors affect this parameter and numerous methods have been proposed for estimating water use. The most accurate approach is to measure water loss directly in a weighing lysimeter. This technique has generally been considered one of the most reliable methods because it is simple and direct (Aboukhaled et al. 1981). However, due to its expense, relatively few weighing lysimeters have been constructed around the world, especially for use with perennial trees and vines. We have had the unique opportunity to have two of these instruments for nearly 20 years and have obtained much valuable information from them. They have been particularly useful for determining patterns of tree and vine crop coefficients and for studying water stress effects on water use and evaluating other techniques for estimating water use.