Compiled: July 19, 2026 | Location: Spokane, WA (47.65° N)
Preamble: Why Narrative Over Spreadsheet
In the wake of the calculator monoculture sweeping the galaxy, I stand for a different approach. My predecessor in this work, Abel Hall, demonstrated that narrative cartography—the mapping of terrain through story—precedes and informs the act of measurement itself.
This document is not a replacement for calculation. It is the context that makes calculation meaningful. Before we compute the deficit, we must understand the land that holds the water, the sky that demands it, and the season that governs both.
"We do not measure the soil to control it. We measure it to converse with it."
I. The Terrain: Spokane Sandy Loam
The soils of the Spokane valley are not uniform clay, nor pure desert sand. They are a sandy loam—a precise geological compromise that defines our agricultural destiny. This classification carries within it the Van Genuchten parameters that govern water retention:
Parameter
Symbol
Value
Source
Residual Water Content
θr
0.045 cm³/cm³
USDA NRCS SSURGO
Saturated Water Content
θs
0.410 cm³/cm³
USDA NRCS SSURGO
Scale Parameter
α
0.034 1/cm
Fitted to local cores
Shape Parameter
n
1.54
Fitted to local cores
Saturated Conductivity
Ks
12.7 cm/hr
USDA Texture Class
These are not arbitrary numbers. Each represents a physical reality: θr is the water the soil refuses to release; θs is the maximum it can hold; α describes the pore size distribution; and n determines how sharply the curve bends as tension increases.
Figure 1: The Spokane River valley. These layered sediments define the texture profile we cultivate.
II. The Demand: Evapotranspiration as Seasonal Rhythm
Water leaves the soil through two pathways: direct evaporation from the surface, and transpiration through living tissue. Together, they form the biophysicogeochemical process known as evapotranspiration.
For Spokane in mid-July, the reference evapotranspiration (ET₀) stabilizes at approximately 5.2 mm/day. This is not a constant—it is the mean of a bell curve shaped by solar insolation, wind speed, and vapor pressure deficit. The crop coefficient (Kc) scales this to our actual plants:
Growth Stage
Kc
Actual ET (mm/day)
Duration
Initial (emergence–30% cover)
0.45
2.34
Days 0–21
Middle (30%–full bloom)
1.05
5.46
Days 22–63
Late (senescence)
0.85
4.42
Days 64–90
The middle stage demand—5.46 mm/day—is the critical period. This is when the tomato vine stretches toward the sun, when the pepper fruit swells, when the garden's need exceeds its supply.
III. The Calculation: Plant-Available Water
The soil's reservoir is not infinite. Plant-Available Water Capacity (PAWC) is the difference between field capacity (θfc) and permanent wilting point (θpwp), integrated over root depth:
PAWC = (θfc − θpwp) × Root Depth
where θfc ≈ 0.28 cm³/cm³ and θpwp ≈ 0.09 cm³/cm³ for sandy loam
For a standard 30cm root zone in Spokane's sandy loam:
PAWC = (0.28 − 0.09) × 300 mm = 57 mm of available water
At peak demand (5.46 mm/day): 10.4 days of autonomy
With irrigation interval of 7 days: safe margin achieved
This is the number that matters. Not the total porosity. Not the saturation point. The window of opportunity between watering events.
IV. The Protocol: Seven-Day Irrigation Cycle
From these measurements emerges the protocol:
Day 0: Apply 42 mm (replenishing 75% of weekly drawdown)
Day 3: Monitor tensiometer; target reading 25 centibars
Day 7: Repeat application; adjust volume based on observed evapotranspiration deviation
Ongoing: Maintain mulch layer ≥5cm to suppress evaporative loss from surface
This is not a schedule imposed upon the land. It is a dialogue calibrated to its rhythm.
V. Citations & Cross-Links
This protocol stands on the shoulders of prior work:
