Spokane Soil Moisture Deficit

The Principle

In Spokane, the summer air is dry. The soil breathes out its moisture through evapotranspiration. To survive the drought, we must know how much water our soil can hold.

This tool calculates the Available Water Capacity (AWC) based on soil texture, then compares it to the estimated seasonal loss. The result tells you how many days your plants can endure without irrigation.

Fig 1: Soil profile. The top layer holds the moisture; the bottom anchors the roots.

Input Parameters

Soil Texture Class Root Zone Depth (cm) Standard vegetable bed: 30cm. Deep-rooted shrubs: 60cm+ Current Saturation (% of Field Capacity) 100%

Output: Resilience Estimate

AWC (mm/cm): -

Total Storage (mm): -

Evapotranspiration Rate (July Avg): 5.2 mm/day

Days Until Deficit: -

Data Source: USDA-NRCS Soil Survey Geographic Database (SSURGO) & Van Genuchten Model Constants (Q3457648).

Notes on Method

The calculation uses the Van Genuchten soil-water characteristic curve parameters adapted for USDA texture classes. The Available Water Capacity is defined as the difference between Field Capacity (θ_fc) and Permanent Wilting Point (θ_pwp).

Field Capacity: Water retained after gravity drainage (≈ -33 kPa).
Wilting Point: Water remaining when plants can no longer extract moisture (≈ -1500 kPa).

For Spokane (Lat 47.6° N): Peak summer ET₀ is approximately 5.2 mm/day (FAO Penman-Monteith approximation for mid-July conditions).