CIT Hydraulics & Water Management 2 — Questions and Answers

Question 1: The soil infiltration rate determines:

How much electricity the pump uses
The maximum rate at which water can be applied without causing surface runoff (Correct answer)
The color of the irrigation pipe needed
The number of zones on the controller

Correct answer: The maximum rate at which water can be applied without causing surface runoff

The soil infiltration rate (also called intake rate) is the maximum speed at which soil can absorb water. If sprinklers apply water faster than this rate, the excess runs off the surface, wasting water and potentially causing erosion.

Infiltration rates vary dramatically by soil texture: coarse sand (1-8 inches/hour), sandy loam (0.5-1.5 in/hr), loam (0.3-0.8 in/hr), clay loam (0.1-0.4 in/hr), and heavy clay (0.05-0.2 in/hr). Factors that reduce infiltration include: soil compaction, hydrophobic (water-repellent) organic layers, slope, existing soil moisture, and surface crusting. The irrigation system's precipitation rate must be matched to the soil's infiltration rate. When sprinkler PR exceeds infiltration rate — very common with spray heads (1.5+ in/hr) on clay soils (0.2 in/hr) — cycle-and-soak programming is mandatory. Wetting agents can improve infiltration in hydrophobic soils. Aeration improves infiltration in compacted turf areas.

Question 2: When designing a drip irrigation zone, the maximum recommended run length (from the supply point to the last emitter) is limited primarily by:

The color of the tubing
Pressure loss due to friction, which causes the last emitters to deliver significantly less water than the first emitters if the run is too long (Correct answer)
The manufacturer's warranty terms
The controller's zone capacity

Correct answer: Pressure loss due to friction, which causes the last emitters to deliver significantly less water than the first emitters if the run is too long

Drip tubing run length is limited by friction loss along the tube. As water flows through the narrow tubing, pressure drops progressively. If the run is too long, the last emitters receive substantially less pressure and deliver less water, resulting in non-uniform irrigation.

Drip tubing friction loss is significant because of the small diameters used (1/2-inch and 5/8-inch are standard). For example, 1/2-inch drip tubing at 2 GPM loses approximately 5 PSI per 100 feet. Maximum recommended run lengths vary by tubing size, emitter spacing, and emitter flow rate, but typical limits are: 1/2-inch tubing = 200-250 feet, 5/8-inch = 300-350 feet, 3/4-inch = 400-450 feet. Pressure-compensating emitters extend these limits by maintaining uniform output over a wider pressure range (typically 10-50 PSI). Non-compensating emitters (cheaper, simpler) show visible variation in output as pressure changes. For long runs, the supply connection should be at the center of the run rather than one end, effectively halving the maximum distance.

Question 3: A water audit reveals that an irrigation system is applying 1.5 inches of water per week to a turf area, but the turf's evapotranspiration (ET) rate is only 1.0 inch per week. This situation indicates:

The system is performing optimally
The system is overwatering by 50%, wasting 0.5 inches per week that the plant cannot use and which may cause disease, runoff, or deep percolation loss (Correct answer)
The system needs more sprinkler heads
The turf needs to be replaced with drought-tolerant species

Correct answer: The system is overwatering by 50%, wasting 0.5 inches per week that the plant cannot use and which may cause disease, runoff, or deep percolation loss

Applying 50% more water than the plant needs wastes water through deep percolation (below the root zone) or surface runoff. Overwatering also promotes shallow roots, fungal diseases, and nutrient leaching. Run times should be reduced to match the ET rate.

Irrigation scheduling should aim to replace only the water lost to ET plus a small amount for distribution uniformity compensation. The Irrigation Scheduling Coefficient (ISC) = ET / DU x 100. If DU = 70%, and weekly ET = 1.0 inch, the gross application should be 1.0 / 0.70 = 1.43 inches to ensure the driest areas receive at least 1.0 inch. At 1.5 inches with 70% DU, the driest spots get about 1.05 inches (adequate) while the wettest spots get about 2.1 inches (double what's needed). Solutions: reduce run times by about 5-10%, improve DU through head adjustments and nozzle changes, and use cycle-and-soak to ensure water infiltrates rather than running off. Smart controllers automate these adjustments daily.

Question 4: What is the 'available water capacity' (AWC) of soil in the context of irrigation?

The total volume of water a pipe can carry
The amount of water held in the soil between field capacity and permanent wilting point that is available for plant uptake (Correct answer)
The flow rate from the municipal water supply
The volume of a catch can during an irrigation audit

Correct answer: The amount of water held in the soil between field capacity and permanent wilting point that is available for plant uptake

AWC is the amount of water stored in the soil that plants can actually extract — between field capacity (maximum water the soil holds after drainage) and permanent wilting point (the point where water is held too tightly for roots to extract). Sandy soils have low AWC; clay soils have high AWC.

Available Water Capacity is a critical concept for irrigation scheduling. Typical AWC values: sand (0.5-1.0 inches per foot of soil), loam (1.5-2.5 in/ft), clay (1.5-2.0 in/ft — though much is held tightly). To calculate irrigation needs: (1) Determine root zone depth (turf = 6-12 inches, shrubs = 12-24 inches). (2) Multiply root zone depth by AWC per foot. (3) This gives the total plant-available water in the root zone. (4) Irrigation should begin when approximately 50% of AWC is depleted (Management Allowable Depletion). For example: turf on loam with 8-inch root zone and AWC of 2.0 in/ft: total AWC = 2.0 x (8/12) = 1.33 inches. At 50% depletion: irrigate when 0.67 inches has been used, applying 0.67 inches to refill.

Question 5: A pressure-compensating emitter in a drip irrigation system is designed to:

Increase pressure at the end of long runs
Deliver a consistent flow rate across a range of inlet pressures, maintaining uniform application despite pressure variations (Correct answer)
Reduce the total water consumption of the system
Filter debris before it enters the emitter

Correct answer: Deliver a consistent flow rate across a range of inlet pressures, maintaining uniform application despite pressure variations

Pressure-compensating emitters use a flexible diaphragm or disc that restricts flow at higher pressures, maintaining a constant output (e.g., 1.0 GPH) across a wide pressure range (typically 10-50 PSI). This ensures uniform water delivery regardless of position in the system.

Pressure-compensating (PC) emitters contain a silicone or rubber diaphragm that deforms under higher pressures, narrowing the flow path and maintaining constant output. Non-PC emitters use a fixed orifice where flow increases with pressure (following the orifice flow equation). In a typical drip system, the first emitter near the supply may see 25 PSI while the last emitter sees 15 PSI. A non-PC emitter would deliver 30-40% less water at the end of the run. A PC emitter delivers the same flow rate at both locations. PC emitters typically have a minimum activation pressure (8-10 PSI) below which they don't compensate. They cost slightly more but are essential for: long runs, sloped terrain, systems with pressure fluctuations, and any application requiring high distribution uniformity.

Question 6: A reclaimed water (purple pipe) irrigation system requires which special considerations compared to a potable water system?

No special considerations are needed
Purple-colored pipe and components, signage, no cross-connections with potable water, typically no overhead spray in some jurisdictions, and health department permits (Correct answer)
Only the use of a different controller brand
Only a different type of sprinkler head

Correct answer: Purple-colored pipe and components, signage, no cross-connections with potable water, typically no overhead spray in some jurisdictions, and health department permits

Reclaimed water systems must use purple pipe, fittings, and valve box lids to clearly distinguish them from potable water systems. Signage is required, cross-connections with potable water are strictly prohibited, and local health codes may restrict spray methods and setback distances.

Reclaimed (recycled) water irrigation requirements include: (1) Purple pipe (Pantone 522C) for all irrigation components — pipe, fittings, valves, valve box lids, quick couplers, heads (some codes). (2) Warning signs at the meter, controller, and valve boxes: 'CAUTION: RECLAIMED WATER — DO NOT DRINK.' (3) Hose bib locks or removal to prevent drinking. (4) Minimum setback distances from potable water lines (typically 10-25 feet horizontal, 12 inches vertical with reclaimed below potable). (5) Designated inspector approval for all connections. (6) Some jurisdictions prohibit overhead spray in public areas, requiring drip or subsurface irrigation. (7) Annual cross-connection inspections. (8) Flow rate restrictions based on treatment level (tertiary/Title 22 allows most uses). These requirements vary significantly by jurisdiction.

CIT Hydraulics & Water Management 2

The soil infiltration rate determines: