Texture - Rough Estimate of H2O Infiltration based on Soil Type
ISC will default to this source useing the most conservative setting available "Clay".
|Texture Drop list||Choose one of twelve soil textures. The summary box is updated to reflect the infiltration rate based on the selected option.|
|Texture description||Depending on which texture item from the drop list is selected, this description will describe the composition of soil types, their relative percentages, and particle size.|
of Agriculture (USDA) Textural Triangle
|The chart identifies each soil type listed in the drop list. The soil textural triangle can be used to determine soil texture from the relative amounts of particles of any two sizes. The composition of a soil type can be interpreted by following the component lines to the appropriate axis. Alternatively, the soil type can be identified knowing the composition of clay, silt, and sand elements.|
|Default Rate Supplied by Parent Resource - This dialog is offered in numerous irrigation setup boxes. By default, ISC will select the parent resource as the source of information for infiltration. The parents value is forwarded to the summary box. Note, the dialog associated with the parent at the top of the hierarchical list, the "System Runtime Limits & Defaults", will indicate "Not Submitted" as it has no parent.|
|Water Infiltration Rate -
LVE recommends this method, because it most closely measures actual
infiltratin rates for the sites soil. The USDA estimate methods will get close to actual, but
for those willing to spend a little time we believe this is the best
The process involved in determining the absorption or infiltration rate can be elaborate. One of the "correct" methods is one sanctioned by the Food and Agriculture Organization of the United Nations, FAO. A website with their official methodology can be found at "Annex 2 Infiltration rates and infiltration test". By their definition "infiltration rate is the velocity or speed at which water enters into the soil. It is usually measured by the depth (in mm) of the water layer that can enter the soil in one hour. An infiltration rate of 15 mm/hour means that a water layer of 15 mm (~ 0.6") on the soil surface, will take one hour to infiltrate."
"In dry soil, water infiltrates rapidly. This is called the initial infiltration rate. As more water replaces the air in the pores, the water from the soil surface infiltrates more slowly and eventually reaches a steady rate. This is called the basic infiltration rate".
To determine the basic infiltration rate they prescribe the use of an "infiltration ring", a device with an outer ring of 60 cm and a concentric inner ring with a diameter of 30 cm. The rings are hammered into the soil to a depth of 15 cm, leaving a 12 cm height exposed above the surface. The space between the two rings is filled with water. The drop in water height relative to the ring top is plotted as a function of time. When the plot becomes linear, the "basic infiltration rate" can be determined using the slope of the linear portion of the plot.
The infiltration rings are available from various sources on the Internet, Google the term "infiltration ring." One interesting link is to a company named "Turf-Tec" that provides a number of options, though apparently targeted for professional use.
That was the official method, but the equipment acquisition is probably too expensive for the average end user. A rough estimate for absorption can be determined using an alternate method. It basically entails digging a 10" deep hole using a post hole digger, filling it with water, and measuring the drop over a period of time. To review this process see "LVE Infiltration Determination".The method described below has been found to provide adequate results.
Dig a hole from the surface to just below root depth. We are only interested in infiltration, not permeability of the soil. The instructions for this procedure are:
1) Use post hole diggers and dig a hole about 10" - 12" deep
2) Use a steel brush to remove sheen on sides of hole
3) Fill with water - allow to empty or soak (1 hr)
4) Refill with water, measure water surface relative to the the top of a board laid across the hole. Record start time.
5) In 1 hour or when measurable dissipation is apparent, measure the water surface as before and record stop time.
6) Enter difference in measured depths under "Water Depth"
7) Enter elapsed time in an HH:MM:SS controls
For best results, soil temperature should be at least 65 deg Fahrenheit for this procedure. Additionally, the soil should contain the approximate moisture content representative of the soil conditions desired. Perform the test when cool temperatures are present in order to minimize evaporation influence.
|Water Depth||The amount of water that was absorbed or that infiltrated into the soil. This depth value is the difference between the height of the water when the clock started, and the height when the clock stopped.|
|UOC drop list||Identify the unit of choice you are working with.|
|Elapsed Time||Enter the elapsed time in the appropriate edit box, hours, minutes, seconds.|
|Summary - whichever source is used, the infiltration rate associated with that source is presented here|
|H2O Rate||The amount indicated here is the value reported any time ISC needs to know the infiltration rate. Select one of the three radio buttons to the right of each category, to select it as the source for use. Alternatively, manipulating any control within a category will automatically select that category as the source.|