iNdUStrY
water transpired from vegetation and evaporated from the soil and plant surfaces. For schedul- ing purposes in turf and landscape, this value is typically a monthly average when corrected for plant species characteristics, and can be used as a baseline to evaluate the plant’s expected water needs in that geographic area.
In our industry I believe this is understood in principle, but oſten misinterpreted in practice. I am amazed by how oſten contractors I meet pro- mote the idea of irrigation scheduling as merely adjusting up/down based on a percentage of the peak season value. As a crude approximation of a direction to aim that is ok, but that’s all!
Hardware limits It is helpful to understand the limitations of irri- gation distribution hardware. For many years we have used fixed sprays, impact heads, rotors and multi-stream nozzles -- spray irrigation hard- ware of various configurations. Tey all share key characteristics and constraints; they irrigate in circles or part circles. But landscapes have straight edges and oſten rectilinear areas, so inevitably there is overspray onto non-landscape or a majority of a given area is over-irrigated, to give the least “overlapped” area an adequate minimum of water.
Soil When spraying water on the surface, above the roots, we seek to flood the surface to the point that water will infiltrate and move down with the force of gravity through the soil’s pore spaces. Tis downward percolation is gravitational water: water in such concentration that it can’t be held by the soil so it moves downward with the force of gravity. If thought about this way, it points out the high potential for waste inherent in spray irrigation. Although plants will certainly take up gravitational water while it is moving, the practical reality is most of the water is wasted to runoff and deep percolation.
Trough scheduling we are attempting to time the “right amount” of water to infiltrate down through the root zone and stop at its lower perimeter.
Putting it all
together Putting all these fac- tors together is to un- derstand key aspects of the soil, water, and plant interaction.
Te water content in soil is not the sole determinant of what is available to plants for their plant water requirement; it is soil particle size and density that deter- mines if water will be available to plants. Te smaller the ma- jority of particles, the more tightly water is held. A sand-sized soil and a clay-sized soil with the same amount of water have completely different amounts of water avail- able to plants. For example:
Sand with 20% water by volume may only bind 5% (water by volume) tightly so 15% is freely available for plant uptake.
In contrast, clay with 20% water by volume will bind water so tightly (because of its extensive surface area of fine particles) that none of the water will be freely available to plants. Terefore plants in that situation may not be able to uptake any water and may begin to wilt.
Tis example emphasizes the importance of understanding soil type and structure to maintain an adequate and consistent moisture level in the root zone. Te water availability defined by soil type is one of the reasons why re- flexively adding/subtracting minutes to irrigation events, as a percentage of ET is a crude method- ology at best.
Looking forward For our industry to grow it may need to embrace irrigation without flooding, or creating excess gravitational water. We need to focus on sup- plying the right amount of supplemental water to what nature may provide through rain
So, if you are using a schedule that keeps your landscape thriving in July, any other time of the year – and particularly in fall as the days become shorter – you are wasting water.
49
UAC MAGAZINE • SEPTEMBER/OCTOBER 2011
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