Calculation Tools
A growing set of web-based tools to help students in ASM 3150/5150 work through irrigation and water management calculations. New tools will be added throughout the semester — bookmark this page.
Chapter 2 — Soil Water
Soil Bulk Density & Porosity
Solve ρb = Ms/Vb and find porosity — useful for compaction checks and water storage capacity.
Soil Water Content & Irrigation Depth
From a soil sample to mass and volumetric water content to an equivalent depth — and how much more water is needed.
Soil Water Depletion & Irrigation Scheduling
Find fraction depleted, fraction remaining, available water capacity, and soil water deficit for a root zone.
Infiltration Depth & Deep Percolation
Find how deep an irrigation application's wetting front penetrates a layered soil, and how much is lost below the root zone.
Soil Texture Water Characteristics
Look up planning-level field capacity, wilting point, and available water capacity by soil texture (Table 2.3).
Chapter 3 — Measuring Water Applications
Unit Converter
Convert between USCS and SI units for volume, flow, length, and area used in water measurement.
Depth – Volume – Area
Solve d = V/A for depth, volume, or area applied — including meter totalizer readings.
Flow – Time – Depth – Area
Solve Q·t = A·d for flow rate, time, area, or depth of application.
Pipe Flow (Continuity)
Solve Q = Vm·A for flow rate, mean velocity, or pipe diameter.
Chapter 5 — Irrigation System Performance
Catch-Can Uniformity
Turn a catch-can test into CU, DU, and the distance-weighted CUH for center pivots — fully flexible number of cans.
Application Efficiency (ELQ)
How much of the water pulled from the source actually ends up usable by the crop — from uniformity, SWD, and runoff.
Scheduling Coefficient
How much longer a turf/landscape system needs to run than a perfectly uniform one would, to adequately water 90% of the area.
Chemical Leaching Loss
Estimate nitrate (or other chemical) lost below the root zone per irrigation, from a uniformity test and soil water deficit.
Canal/Ditch Seepage Loss
Estimate seasonal seepage and vegetative losses from an unlined delivery ditch or canal.
Peak Daily ET & Net Capacity
From allowable depletion to a planning-level peak daily ET (Table 5.3) to gross system capacity.
Gross Capacity & Farm Well Discharge
Size a well/pump to serve any number of fields, accounting for inefficiency, downtime, and conveyance losses.
Irrigation Scheduling (Sensor-Based)
Soil Moisture Sensor Irrigation Scheduling
Turn readings from any number of soil moisture sensors into a when-and-how-much irrigation decision — no soil or crop lookup required.
Center Pivot Design & Operation
Center Pivot Rotation Time & Application Depth
How long a pivot needs to run to apply a target depth of water — or how much depth it lays down in a given run time.
External Resources
AgriMet Real-Time Crop Water Use
Live crop ET data delivery for irrigation scheduling. Opens in a new tab (agrimetcropwateruse.netlify.app).