Irrigation Water Quality & Quantity

Why the water comes first

The crop's demand is a number; the source's supply is the constraint. A system designed to the crop's demand but not the source's supply is a system that fails in the dry weeks — and the dry weeks are the ones the farm is paying for. The assessment's four questions, in the order they decide the design:

1. Quantity — the dry-season yield

The number that matters is the source's yield in the dry season, not the average week. The average week is the wet season's number, and the design built on it is a design that fails in the months the rain doesn't come. By source:

• The borehole. The yield in litres per hour — measured, not assumed (the "the borehole is about this" is the number that undersizes the system). The assessment is where the measurement happens; the pump is specified from the yield and the lift, and the crop's peak week is the demand the yield must meet.
• The dam. The capacity (the litres held) and the seasonal behaviour (what it holds into the dry months). The storage check is the design line: does the dam carry the crop's peak week, or does the system need the tank or the second source to bridge the gap? The quantity guide's storage decision is the answer.
• The river / the lake intake. The available flow at the intake, and its seasonal behaviour — the low-flow season is the number, not the flood season. The filtration is the quality side; the legal position (the water rights, the permits) is the farm's to hold, and the design works from the water the farm is entitled to.
• The tank (the harvested rain). The capacity, and the refill source. The tank's size is the crop's peak week plus the margin — the Nairobi plot's week, the strawberry bed's week, the watermelon block's fill period. The size is the design line, not the guess.

2. Quality — what the water carries

The quality decides the filtration, and the numbers that matter are the four:

• The sediment (the silt, the sand). What the water carries in suspension — the dam's and the river's signature load, the seasonal silt's draw. The sand/media filter is the component; the filtration guide is the matching. The jar test (the water left to settle, the sediment's layer measured) is the farmer's own quick read.
• The algae / the biological. The lake and the warm dam's signature load — the biological film that grows in the slow water, the algae season's draw. The filtration is the physical check; the chlorine treatment (the line cleaning guide) is the reset when the growth is established.
• The iron. The borehole's signature load — the iron that clogs the emitters slowly over the months, the "the system is gradually failing" that's the iron's signature. The disc filter's rinse on the calendar is the routine; the jar test (the rust colour after the rest) is the tell. The filtration guide covers the iron's story.
• The salinity (the salt). The coastal aquifer's signature load — the salt that the crop's tolerance is the limit for. The quality number is the EC (the electrical conductivity), and the crop's tolerance is the match: the citrus and the mango carry more salt than the strawberries, and the Mombasa and Kilifi pages note the coastal's salinity as the design's first question. The harvested rain (the tank) is the coast's clean supply, and the tank system is the water strategy where the aquifer is the brackish one.

The assessment measures the four — the assessment is the visit where the jar test, the intake's position, the seasonal behaviour and (where the farm has it) the lab's read come together. The design works from the measured numbers, not the source's type. The filtration guide is the quality side's setup; this guide is the quantity side's method.

3. Position — the head

The height of the source relative to the field is the number that decides gravity or pump. The rule of thumb: roughly 1 metre of height gives about 0.1 bar — the tank 10 metres above the field gives about 1 bar at the base, and the how drip works guide walks the pressure. The practical read:

• The gravity works where the head is enough for the line's working pressure (the ~1 bar the standard drip line wants) plus the field's friction. The spring on the hillside, the tank on the ridge, the dam above the field — the gravity is the cheapest pump on the farm, and it's why the source's position is a design question, not a footnote.
• The pump is the answer where the gravity can't reach — the borehole below the field, the dam below the field, the flat ground. The pump is specified from the flow and the lift (the litres per hour the field needs, plus the metres the water must rise), and the design guide walks the calculation.
• The two together — the gravity for the base, the pump for the boost. The mixed system, where the head is part of the answer and the pump is the rest.

4. The storage decision — carrying the gap

The source's dry-season yield and the crop's peak week are two numbers, and the gap between them is the storage decision. Where the yield carries the peak, the storage is the margin (the interruption's buffer, the week the source pauses). Where the yield doesn't carry the peak, the storage is the bridge — the tank, the dam's second source, the harvest that carries the dry. The cost guide notes the storage as the head's cost line; the assessment is where the two numbers are measured and the decision is made.

• The tank. The stored water — the harvested rain (the Nairobi plot's supply, the strawberry bed's buffer), the pumped storage (the borehole's week held for the peak). The size is the crop's peak week plus the margin.
• The dam's role. The rainy-season collector, the dry-season question — the Machakos and Makueni drylands' water strategy, the storage check the design line.
• The second source. The borehole plus the tank, the dam plus the river — the mixed supply, where the gap is too big for one source and the two together carry it. The design guide walks the mixed-source layout.

The assessment, at a glance

The farm's own quick read

• The jar test. A jar of the water, left to settle: the sediment's layer is the load, the rust colour is the iron, the clear water is the clean source. The farmer's own quality read, before the assessment confirms it.
• The dry-season question. What does the source do in the dry months? The dam's level, the borehole's yield, the river's flow — the dry-season number is the design's number, and the farmer's memory of last dry season is the starting point the assessment measures against.
• The head's question. Is the source above the field, and by how much? The rough read (the metres, the 0.1 bar per metre) is the gravity-or-pump answer, and the assessment is where the exact number is taken.