Page 24 - Landscape-Issue141
P. 24

IRRIGATION FEATURE


          UNDERSTANDING WATER

          DISTRIBUTION IN SOIL:


          DRIPPER SPACING AND DELIVERY RATE



          By Jovan Erasmus, Agronomist, Netafim SA

          In the precision planning process of a drip irrigation system, one of the crucial steps
          is to determine the correct delivery rate and dripper spacing that will be required

          for optimal irrigation for your conditions.


























          Each type of soil has a unique texture and structure

             o determine the required delivery   horizontally by capillary action. Hydraulic   and  the  water  will  drain  past.  Soil  with
             rate and dripper spacing, we need   conductivity  is  the  water’s  potential  to   larger particles has lower porosity and a
          Tto understand the way water is    move  in  soil.  Water  moves  on  a  gradient   lower surface to volume ratio. Clay type
          distributed in soil and what affects water   from wetter to dryer soil, and the wetter the   soils with smaller soil particles have
          distribution in soil. Each type of soil   soil, the easier water will move through it,   higher porosity with a higher surface to
          has a unique texture  and structure  and   as conductivity is higher. If soil is dryer, the   volume ratio. More porous soils allow
          these physical characteristics affect soil   empty pores in the soil must be filled first,   better water-air balance management in
          infiltrability,  hydraulic  conductivity,  water   and water distribution is slower.  the soil and create space for the water to
          retention and porosity. All of these factors                          move through the soil.
          represent important processes that work   Water retention
          together holistically to determine the   Water retention is the soil’s ability to hold   Saturated circles
          movement of water in soil.         water and nutrients. This greatly depends   When crops are drip irrigated, the water
                                             on the size of the soil particles. Soils with   is delivered in one spot, and we rely on
          Infiltrability                     smaller particle sizes offer better water   the  soil’s  physical  ability  to  distribute
          Soil infiltrability is the rate at which water   retention. Sandier soils generally have   the water. A wetted bulb or saturated
          can be absorbed into the soil and is mostly   larger particle sizes compared to heavier   circle forms under the dripper. Your soil’s
          determined by soil texture or the percentage   clay  soils,  which  is  why  clay  soils  have   physical characteristics and the delivery
          of sand, silt and clay in soil. It is important that   better water retention than sandier soils.   rate of the dripper will determine the
          the delivery rate of your irrigation system is   Better water retention simply means that   dimensions of this circle.
          balanced with the soil’s infiltration rate. If the   you can put more water into the soil and   Roots require oxygen to grow and take
          water is delivered at a rate higher than the   that it will hold the water for longer.  up water and nutrients, and it is therefore
          rate at which it is absorbed into the soil, there                     important that there is balance between
          tends to be greater horizontal spread of water   Porosity             the  water and air  that  fills soil pores. As
          close to the soil surface. This brings risks such   Soil’s  porosity  depends  on  the  spaces   water infiltrates soil, it replaces air in the
          as forming of the shallow root-zone, losing   between  soil  particles:  the  macro  and   pores.  In  drip  irrigation,  this  creates  a
          water off the ridges in orchards, ponding, run-  micro pores. These are the spaces where   gradient between low and high oxygen
          off and higher evaporation. In short, water-  water and nutrients are stored and where   levels in the soil. Drippers with higher
          use efficiency will be negatively affected.  the roots will grow.     delivery rates displace more air faster,
                                              Macro pores allow easy movement   creating  variation  in  the  soil’s  oxygen
          Hydraulic conductivity             of  water  and  air  through  soil. Water  will   levels and leading to lower uptake of
          Water  moves  through  soil  as  it  is   infiltrate the soil until these pores are   water. Drippers with lower delivery rates
          pulled down by gravity and distributed   filled, at which point it reaches saturation   allow for better hydraulic conductivity


          22    Landscape SA • Issue 141  2024                           Check us out www.salandscape.co.za
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