Part of Irrigation
Water assessment is the systematic evaluation of available water sources to determine their suitability, volume, reliability, and accessibility for irrigation. Before building any canal, ditch, or water-lifting device, a community must understand what water it actually has — not just whether a stream exists, but how much water flows in the dry season, how high the water table sits in summer, and whether the water’s mineral content will harm crops over years of use. Irrigation based on miscalculated water supply fails at the worst possible time: mid-growing season when crops cannot be saved.
Why Assessment Comes First
The history of failed irrigation schemes is largely the history of optimistic assessment. Communities build canals to rivers that run dry in summer, dig wells that fail after five years of pumping, or divert streams that other communities depend on downstream. Thorough water assessment prevents these failures.
A complete assessment answers five questions:
- How much water is available? (volume and flow rate)
- When is it available? (seasonal patterns)
- How reliably is it available year to year?
- How accessible is it? (elevation, distance, infrastructure needed)
- Is it suitable for irrigation? (salinity, sediment, pH)
Surface Water Sources
Rivers and Permanent Streams
Permanent streams are the most valuable irrigation source because they provide continuous flow during the growing season.
Assessment parameters:
Flow rate: Measured in cubic meters per second (m³/s) or, more practically, liters per second (L/s) or liters per minute (L/min). See the companion article on Flow Measurement for methods.
Seasonal variation: Measure flow at several points during the year, especially at the expected low-flow period (late summer / end of dry season). Many streams that flow well in spring have dramatically reduced flow by August. See Seasonal Variation.
Diversion elevation: Can the water be diverted by gravity into the fields? Check whether the stream bank is higher than the field level at a suitable diversion point. Even a 30 cm elevation advantage allows gravity-fed canals.
Flood risk: Examine the stream banks for high-water marks (debris lines, soil staining, flood scour on trees). Irrigation infrastructure placed in the flood zone will be destroyed periodically.
Water quality indicators (field-observed):
- Clear, fast-moving water over rock or sand = generally good
- Clear water over fine clay = possibly high mineral content
- Brown turbid water during storms = high sediment (beneficial in moderation, problematic if it deposits and clogs canals)
- Foul odor, milky color, or foam = possible contamination — assess before using
Seasonal Streams and Ephemeral Watercourses
Streams that flow only during and after rains can still support irrigation through:
- Flood recession irrigation: Plant fields in the moist soil left after floodwaters recede (practiced in the Nile valley and Mesopotamia for millennia)
- Runoff harvesting: Direct storm runoff into retention ponds during rains, then distribute from the pond during dry periods
- Spate irrigation: Direct flood pulses into broad shallow basins, allowing water to spread across fields and infiltrate before runoff escapes
These strategies require no permanent water source — only reliable seasonal rains.
Ponds and Lakes
Natural ponds and lakes buffer seasonal variation — water stored during wet periods is available during dry periods.
Assessment for irrigation use:
- Surface area and estimated depth: Volume = area × average depth. A 1-hectare pond averaging 2 m deep holds approximately 20,000 m³ of water — enough to irrigate roughly 2–4 hectares of vegetables through a dry season.
- Replenishment rate: How much water enters the pond from rainfall, springs, or stream inflow? A pond that receives less water than irrigation demand will drop and eventually empty.
- Seepage rate: Clay-bottomed ponds lose little water; sand or gravel bottoms lose substantial water. Dig a small test hole and observe how fast water seeps away.
Groundwater Sources
Springs
Springs are the highest-quality irrigation water source in most situations — naturally filtered, temperature-stable, and often flowing continuously.
Locating springs:
- Vegetation signals: dense lush growth (willows, alders, cattails, sedges) in otherwise dry terrain indicates shallow groundwater or surface seeps
- Slope breaks: springs often emerge where impermeable rock layers intersect hillsides, causing groundwater to surface
- Soil color: persistent dark, wet soil on a slope or at the base of a cliff
- Animals: animal trails converging toward a point in dry terrain
Spring assessment:
- Measure discharge flow (bucket and stopwatch — see flow measurement)
- Test whether flow is consistent across seasons (springs fed by deep aquifers are more stable than those fed by shallow soil moisture)
- Check whether spring is on the upslope side of intended fields — gravity distribution is possible
Wells
A well accesses groundwater below the surface. Well viability depends on:
Depth to water table: Dig a test pit 30–60 cm wide and observe how deep before water begins seeping in. In humid regions, the water table may be 1–5 m deep; in arid regions, 10–50 m or more. Deeper wells require more infrastructure.
Recharge rate: After digging to the water table, observe how fast the pit fills. A pit that fills slowly (takes hours) has low recharge — pumping faster than the recharge rate lowers the water table and eventually empties the well. The sustainable yield of a well is its recharge rate, not its total stored volume.
Aquifer type:
- Unconfined aquifer: Water table rises and falls with rainfall. Vulnerable to drought and over-extraction.
- Confined aquifer: Water is trapped under impermeable rock layer under pressure. May flow freely when tapped (artesian well). More reliable but limited recharge.
Water Quality Assessment
Salinity: High salt content (sodium, chloride) damages soil structure over time and stunts or kills crops. Field test: taste a small amount — distinctly salty water is usually unsuitable for long-term irrigation. A simple tasting test distinguishes fresh (below 500 ppm total dissolved solids), brackish (500–3,000 ppm), and saline (above 3,000 ppm). Most crops tolerate up to 1,000–1,500 ppm for several seasons; some salt-tolerant crops (barley, cotton, beets) tolerate up to 5,000 ppm.
Sediment load: Moderate sediment (turbid but not muddy) deposits mineral-rich silt on fields — beneficial. Heavy sediment load clogs canals and can bury seedlings. Allow heavily turbid water to settle in a basin before distributing to fields.
pH: Most crops grow well in irrigation water between pH 6–8. Water from limestone bedrock may be alkaline (pH 8–9). Water from acidic peat bogs may be acidic (pH 4–5). Both extremes can cause nutrient lockout in soil. pH can be estimated with litmus paper or observed from plant growth — if native plants grow vigorously around the water source, pH is likely acceptable.
Biological contamination: Water sourced below animal grazing areas or human settlements should be assumed contaminated with pathogens. For drinking water this is unacceptable; for irrigation of root vegetables or leafy crops eaten raw, contaminated water poses health risks. Allow contaminated irrigation water to evaporate from soil surfaces before harvest, or irrigate at the base of plants rather than on edible portions.
Watershed Assessment
A watershed is all the land that drains to a given stream or river. Understanding the watershed helps predict:
- Total water available (watershed area × average rainfall × runoff coefficient)
- Upstream human and animal impacts on water quality
- Future reliability as land use changes
Rough watershed water budget:
- 1 ha watershed in a 600 mm/year rainfall region produces approximately 150,000–300,000 L of runoff annually (depending on soil, vegetation, slope)
- 1 ha of irrigated vegetables requires approximately 4,000–6,000 m³ (4–6 million liters) per year
A community with a 100-hectare watershed receiving 600 mm annual rainfall can expect 15–30 million liters of runoff — theoretically enough to irrigate 3–7 hectares intensively. This is a rough planning figure only; actual diversion efficiency, seepage losses, and seasonal distribution reduce practical usable volume significantly.
Documentation
Record all assessment findings in a water log:
- Date, weather conditions, and recent rainfall history at time of measurement
- Location of measurement (landmark description or simple map)
- Flow measurement result and method used
- Observations on color, smell, turbidity
- Water table depth if wells or test pits were dug
- Seasonal data points (record the same source at multiple seasons)
Without records, communities make decisions based on what people remember — often the wettest year, not the driest. The driest year is what irrigation planning must survive.
Water assessment is the foundation on which every other irrigation decision rests. A community that knows its water — how much it has, when it flows, and how reliable it is — can design irrigation infrastructure that will serve for generations. A community that guesses will build infrastructure for the water it hopes to have rather than the water it actually has.