Evapotranspiration and Irrigation Scheduling in Fruit Trees

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Evapotranspiration and Irrigation Scheduling in Fruit Trees

 Introduction

 Water is an essential component of crop productivity. Even in humid climates, natural precipitation is not sufficient to meet plant water requirements, so additional water is applied through irrigation.  To optimize production, irrigation must be applied at the proper rate to replenish only that which the plant needs.  Over-irrigation can drown a plant, leach fertilizer out of the rooting zone, and waste water and energy while under irrigation may result in water stress and reduced yields.  Therefore determining how much water the plant actually needs will save resources and improve irrigation efficiency. 

 Irrigation Scheduling

 Use of a management scheme to schedule irrigation ultimately leads to more consistent crop yields and quality, and more efficient use of a valuable natural resource. A commonly used method often referred to as a ‘checkbook’ strategy with the sources of water, precipitation and irrigation, being additions to the checkbook and three major sinks or losses of water: evapotranspiration (the combined loss of water from crop transpiration and soil evaporation), runoff, and drainage of water out of the crop rooting zone.  Mathematically, this scheme relies on the balance: 

PPT  +  IRR  =  ET  +  RO  +  DR

 Where PPT is precipitation, IRR is irrigation, ET is evapotranspiration (combined loss of water to plant transpiration and soil evaporation), RO is runoff, and DR is drainage.  Of these variables, PPT and IRR are relatively easy to monitor with precipitation gages. The RO term varies with soil texture and slope.  If properly managed runoff can be minimal.  Drainage can occur whenever the actual soil moisture value exceeds field capacity (e.g. such as when a heavy rainfall event occurs on a wet soil).  The soil texture is important as it affects the infiltration capacity and how much water the soil can hold for use by the plant.  ET is a function of both atmospheric conditions and physical characteristics of the crop system itself.

 Evapotranspiration

Plants need water to replace that which evaporates from the soil surface and transpires from the plant itself.  From the atmospheric side, there are four major factors involved in ET:  1) the predominant source of energy to evaporate water is sunlight, so the more solar radiation the greater ET; 2) the warmer the air, the more water vapor it can hold and thus the greater the potential for ET; 3) the drier the air (the less water vapor it is already holding), the greater the potential for ET; 4) the greater the wind, the greater the ET.  In a humid climate such as Michigan’s, the solar radiation and air temperature terms dominate ET on a daily basis.

 Transpiration is the loss of liquid water from a plants tissue as vapor that is released into the atmosphere.  Water vapor is lost through the stomata on leaf surfaces that the plant can open and close.

ET can be estimated in a variety of ways. One technique that is accepted as an international standard is the Penman-Monteith Equation, which is used to obtain what is called the reference ET (ETo) with detailed weather data.  ETo assumes a 4-inch grass-covered surface that is well-watered and unshaded. The actual ET of a crop at any given time depends on the amount of leaf area and the developmental stage, so the reference ETo values must first be multiplied by a crop co-efficient to determine the crop specific ET value.  

 The crop coefficient is a multiplier index that relates ETo to a specific crop at a particular time in its development.  Kc changes with crops as they grow, e.g. the Kc of fruit trees increases rapidly in the spring as the trees leaf out to full canopy. To estimate ET, the reference ET is multiplied by the crop coefficient Kc to determine the actual water lost from the crop via ET:

 

Development of a seasonal Kc curve representative of tree fruit production systems in Michigan will lead to improved water use when using irrigation scheduling.  Research at MSU funded by Project GREEEN has focused on determining actual plant water use through assessment of soil moisture and evapotranspiration.  Preliminary calculations suggest that higher values of Kc compared to values currently used may be justified.  Fact sheets summarizing the results of this study will be released in the near future. 

Instrumentation of at the MSU Horticulture Station in Traverse City, MI.

 

For more information on irrigation scheduling and weather information in your area, use the following links. 

 FAO Irrigation and Drainage Paper No. 56                      http://www.kimberly.uidaho.edu/ref-et/fao56.pdf
MSU Enviroweather Website                                            http://enviroweather.msu.edu/homeMap.php

 Staff working on this part of the GREEEN project include Jeff Andresen, Dept. of Geography, Michigan State University (MSU), Sabah Dawood, Jennifer Jury, Steve Marquie, and Steve Miller Department of Biosystems and Agricultural Engineering, MSU.

Contacts: Jeff Andresen, Dept. of Geography, Michigan State University (MSU) andresen@msu.edu

Steve Marquie, Department of Biosystems and Agricultural Engineering, MSU, wb9tow@egr.msu.edu

Steve Miller, Department of Biosystems and Agricultural Engineering, MSU mill1229@msu.edu

 Some of the material in this information sheet adopted from:  Andresen, J.A., 2007. Irrigation of Fresh Market Tomatoes.  Proc. 2007 Great Lakes Expo, Michigan Horticultural Society, Grand Rapids, MI, 3-6 December 2007.