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Colorado State University Master Gardener Soil Water Holding Capacity and Irrigation Management Page 3 of 7 <br /> <br />Evapatransplratfon, ET is the rate a crop uses water for transpiration plus evaporation from the soil surface. <br />Primary influences on ET include weather factors (solar radiation, temperature, wind, and humidity) and the stage <br />of plant growth. <br />On hot or windy days, ET will be higher; on cool, humid days, ET will be lower. To illustrate seasonal variations, <br />the typical evapotranspiration rates for cool season grass (ET) along Colorado's Front Range are found In the <br />Table 1.. <br />Table 1. Typical ET for Cool Season Turf along Colorado's Front Range <br />April May acne July August September October <br />[Per month 3.88" 4.93- 6.04" 6.52" 1 1 5.47" 4.01" 2.76" <br />Per day 0.13^ 0.16" 0.20" 0.21 0.18" 0.13" 0.09" <br />Source. Northern Colorado water Conversancy District. <br />Local ET data is available on the web at www,coloradoet.org, or form many tocal water providers. <br />I2oc?ting x}??'t?1 ._,_____._..^r_._w?Y...._.._?_._?._.._.-....._ _ <br />Rooting depth is also another primary factor influencing irrigation management. Kgotti.9aly grow where thgrg are <br />adgamAte lc%te)s_.of Soil. oxygen. In clayey or compacted soils, where a lack of large pore space restricts oxygen <br />levels, roots will be shallow. Plants with a shallow rooting depth simply have a smaller profile of soil water to use. <br />(Figure 31 <br />t ^? <br />,v <br />Figure 3. Plants with a deeper rooting system reach a larger supply of water and can go longer <br />t k <br />between irrigations. In compacted or clayey soils, tow levels of soil oxygen limit rooting depth, <br />t q (rK reducing the supply of available water. <br />Plants Ability to Extraet Water <br />Plants vary in their ability to extract water from the soil, For most plants, the available water is about 50% of the <br />soil's total water supply before reaching the permanent wilting point. Onions are an example of a crop that can <br />only extract about 40%. <br />A similar, 4„rr t unr__Igteq, Issue is the plant's ability to survive on dry soil (drought mechanism). Many plants, like <br />impatiens, readily wilt as an internal water conservation measure. Some plants, like cacti, have an Internal water <br />storage supply and a waxy coating. Trees close the stomata in the leaves, shutting down photosynthesis, during <br />water stress. <br />CMG Home I Ask a Colorado Master Gardener I CMG Gard^.nNotes I Home Gardening Publications <br />http://Www.ext.colostate.edulmg/files/gardennotes/261-SoilWater.html <br />1/3/2008 <br />66