2.1.2 Part 3: Soil Moisture & Seasonal Water Balance

What soil properties determine its water holding capacity?

Soil texture (proportions of sand, silt, and clay), soil structure, and organic matter content.

Why do clay soils retain more water than sandy soils?

Clay has smaller pore spaces and higher organic matter, creating greater capillary pull to retain moisture.

Define soil field capacity.

The maximum amount of water a soil can retain after excess gravitational water has fully drained away.

Define plant wilting point.

The threshold where soil moisture is so low that plant roots can no longer extract water, causing plants to wilt.

How does antecedent soil moisture affect flood risk?

Saturated soil reduces further infiltration to zero, forcing all additional rain into rapid surface runoff, creating higher flood peaks.

What soil moisture threshold was needed to trigger deep groundwater recharge in tested karst regions?

A volumetric soil moisture content of around 35%.

What is baseflow?

The sustained, steady flow of a river during dry periods, supplied primarily by slow groundwater discharge.

Write the mathematical equation for drainage basin Water Balance.

P = Q + ET ± ΔS (where P = Precipitation, Q = Runoff/Discharge, ET = Evapotranspiration, ΔS = Change in Storage)

What is a "water surplus" in the UK water balance, and when does it occur?

When Precipitation exceeds Evapotranspiration (P > ET)

What is a "water deficit" in the UK water balance, and when does it occur?

When Evapotranspiration exceeds Precipitation (ET > P)

Contrast average July vs December water balance statistics in Southern England.

July: Precip = 40-60 mm, PET > 100 mm (Deficit). December: Precip = 80-100 mm, PET < 20 mm (Surplus).

What is soil moisture recharge?

The replenishment of soil moisture stores occurring in autumn when precipitation rises above falling evapotranspiration rates.