Soil MoistureTe mākū o te oneone

Hawke’s Bay Regional Council monitors soil moisture content at several climate stations across the region. The soils monitored are those found at the climate stations and may be different from surrounding agricultural soils. However, this data is of use across the Bay as it provides a generic indication of regional trends.

Soil moisture data is collected using Aquaflex soil moisture sensors. The sensor is a long flexible tape (3m length) which is buried in the root zone on an angle from approximately 0mm down to 300mm soil depth. Units are percent soil moisture content.

How do we use soil moisture information?

Soil moisture information is particularly useful for assessing irrigation needs for a variety of crops. It is also used for analysis of long-term climate trends, measuring how often plant growth is restricted by soil moisture, and providing an indication for early intervention and drought management decisions.

Soil moisture conditions can influence river flows, so when the soil moisture deficit is high, the soils absorb more water and reduces the risk of flooding.

Soil moisture is also useful for identifying when not to irrigate effluent, to avoid surface runoff into streams.

Soil moisture levels for irrigation and crop management

Successful irrigation means applying sufficient water to avoid a reduction in plant yield due to water stress, while not producing a saturated soil or wasting water through an excessive application. The aim of most irrigation is to keep soil water levels between field capacity and stress point.

In order to better understand soil water availability to plants, common terms are described below, and presented graphically in the following figure:

Saturation:

Saturation occurs when all the soil pores are full of water. Saturation commonly occurs in winter and spring immediately after heavy rainfall.

Field capacity (FC):

Field capacity is the maximum amount of water the soil can hold (expressed as a percentage of soil volume), or alternatively, the water content of the soil after excess water has drained away (approximately two to three days after heavy rain). Field capacity is the upper limit of plant-available water. For irrigation purposes, the field capacity should not be exceeded so that wastage of water, energy and nutrients is avoided.

Stress point (SP):

The point where the roots cannot extract water at the rate required, so the plant will be under 'stress'. Stress point is also known as the refill point, and is approximately half way between Field Capacity and Permanent Wilting Point.

Permanent wilting point (PWP):

When the soil moisture falls to this level, plants have wilted and will cease to grow. Soil moisture deficit: The amount of rainfall (mm) required to return the soil to Field Capacity.

Readily Available Water (RAW):

Readily Available Water is the range between Field Capacity and Stress Point where water is available to plants. It is possible to calculate the number of days before stress point is reached, and therefore irrigation rotations, in the following publications: Information on Understanding Soil Moisture, Scheduling Irrigation and Irrigation Efficiency Evaluations are located in our Environment Topics publications.