Booklet 4: Irrigation Essentials

Booklet 4: Irrigation Essentials

Introduction

This booklet is the fourth of the series of six booklets providing a comprehensive resource for irrigation managers and operators in New Zealand.

This booklet covers the background resource information needed to successfully design, install and manage irrigation. It includes - soil water properties, climate and weather, and the crop being grown.

Figure 1 summarises the interaction of soil, climate and the crop – a field scale irrigation water cycle.

Figure 1: Irrigation water cycle

Soils

1

The Importance of Soil

SThe soil iacts as a short-term ‘water storage’ for plant growth. Plants access the water held in the soil to allow them to continue growing between rainfall or irrigation events. Soils vary greatly depending on their make-up and origin. Tand the amount of water storage stored and th eir interaction with water also a soil’s ability to receive and release varies itwater also varies.

Much of New Zealand has very diverse and changeableintermingled soil types. Frequently They can changes from deep to shallow, and coarse to fine textures are found over very short distances. For successful irrigation, All a propertiey’ssoil information soil characteristics must be verified (mapped) at farm scale (1:5,000)on-site. PThe scale of most public soil maps are usually mapped between 1:50,000 – 1:250,000. These provide a guide to whatis soil types you may find on your property but should not be used literallyare not sufficiently detailed for irrigation either 1:50,000 or 1:250,000. This is only sufficient for broad-acre farms on very uniform soils. Soil water sensingor technologies,y such as Electro Magnetic Conductance (EM mapping ),) are becoming morenow commonlyreadily available. These allows extremely detailed soil mappingng but still require a degree of ground-truthing.

Soil Properties

Soil is made up of a mixture of mineral matter (soil particles) matter, organic matter, water and air. For example, aA typical Lloam soil is made up of 45% mineral matter, 5% organic matter, 25% water, and 25% air.

Air and water occupy the pore spaces in soils. Pore spaces are the voids (holes) between the soil particles. Soil has both macro and micro pores. The macro-pores act as drains within the solil, allowing water to enter and flow through it. The micro -pores act as water and air stores.

Air and/or water occupy approximately half the volume of soil. Fine-textured soils have more total pore space than coarse -texturesextured soils. As soils absorb water, the air space decreases.

For irrigation, the main soil properties of interest are:

·  Texture

·  Structure

·  Depth

·  Water Infiltration RRate

·  Drainage Ccharacteristics

·  Upward Wwater Mmovement (Ccapillary Rise) from water tables below the root zone.

Texture

1

Texture is important as it influences athe soil’s water properties of:

·  Water holding capacity

·  Drainage characteristics

·  Water i Infiltration rRate

·  Upward water movement (capillary rise) of water

Soil texture describes the particle sizes in soil. Particles are grouped as

1.  clay (<0.002mm)

2.  silt (0.002-0.05mm)

3.  sand (>.0.05mm)

Soil texture description is based on the fractions of particle sizes present. The soil texture triangle in Figure 2 (below) shows soil texture names with the proportions of sand, silt and clay in each.

Figure 2:1 Soil texture triangle labels soil textures according to proportions of sand, silt and clay

For example, the blue arrows in Figure Figure 12 show a soil with 3050% sand, 5020% silt and 2030% clay. It fits within the “sandy clay loam” description.

Methods for determining soil texture are provided in the extension resources.

Soil Pprofiles and Hhorizons

A soil profile is composed of successive layers known as horizons (Figure 32).

Many New Zealand soils are formed from alluvial (river borne) or loess (wind borne) deposits. Soils may have many horizons, each laid down at different times with different materials. Because of this, the texture of each horizon needs to be determined. This in turn will determine the characteristics of the whole soil profile.

Organic Mmatter

Typically, the upper most layers or horizons have the most organic matter. Organic matter is important as it improves soil structure by acting as a binding agent. It causes soil to clump together forming soil aggregates. It Organic matter can hold up to 90% percent of its weight in water and releases nearly all of it for plants to uptake. The combination of these attributes improves the soil's ability to take- up and hold water.

Soil Structure

Structure refers to the arrangement of, and connections between, soil particles. Soils made up of practically all sand or all silt do not show any appreciable structural arrangement because of a lack of the binding properties provided by clay.

Structure is a function of the texture and organic matter. Soil structure influences many important soil properties:, such as the rate of water infiltration, water holding capacityretention, aeration, and drainage, because of its effects on pore size and connectivitydistribution. Well-structured soils have a networks of inter-connecteding pores which enable water to infiltrate and drain easily. Compacted soils have fewer and unconnected pores spaces for the water resulting in a slowerand the flow rate into and through it is slower.

Soil Depth

A combination of texture, structure and depth determine aprovides the soil’s water holding capacitystorage capacity. Generally, theThe deeper thethe soil the greatlarger its the water holding capacitystorage, however . mMany New Zealand soils have pans (permanent or seasonal) or other impermeable layers that inhibit or exclude water and plant root penetration. Also, some soils frequently become water logged, particularly during the winter months. MottlThe anaerobic (no air) conditions that water logging creates limits or reduces rooting depth.ed Water logged soils affect rooting depth etc ...... Andrew

Soil Drainage and Permeability

A soil’s permeability and Ddrainage characteristics control the rate of water movement through it (permeability) and out of it (drainage) of the soil.

Generally, coarser, well-structured soils have better permeability and drainage properties. Rain and irrigation entering (infiltrating) the soil can easily move through it, filling it to capacity and draining the excess.

Key features that indicate problems with permeability and drainage are brown and grey mottling (a sign of extended wetness) or very grey or pale soils (a sign of water logging). Hard and compacted layers or very fine textured layers can cause slow movement. Anything that stops root penetration reduces the plants’ access to soil moisture (and nutrients).

Water logging from slow permeability and drainage, or high water tables make s plants more drought prone by reducing their effective rooting depth.

Upward Capillary Flux

Capillary forces in the soil allow water to move upwards against the force of gravity. Fine pores pull water from wetter (high water potential) to drier (low water potential) zones.

The effect is that water from below the root zone can move up from the water table and supply crop needs. Soil moisture measurement is the only way to monitor what is happening if your soil is subject to capillary rise.

Water Holding Capacity (WHC)

Soil holds water like a sponge. It soaks it up until it can hold no more, then it drains. The total amount that can be stored is the Water Holding Capacity (WHC) of theat soil.

WHC It is dependent on texture, and structure and depthral arrangement of the soil. Coarse textured sandy and gravelly soils have low water storage and fast drainage. Silts and clays retain more water and drain slower.

WHC is usually expressed in millimetres (equivalent to rainfall) held per depth of soil (mm/100mm), but can also be expressed inrather thanwhich is the same as percentage soil moisture. For example 25mm (water) per 100mm of soil is equivalent to 25% soil mosituremoisture.

Water Hholding Ccapacity Pproperties

Figure 4 shows how soil Wwater is held in the soil in the macro and micro pores between the soil particles. Water is held in the pores by capillary action,; the smaller the pore, the tighter it i’s held. When all the pores are full the soil is saturated. If more water is added it drains out under the force of gravity. This is drainage.

After a day or two, the macro- pores empty. Micro- pores are able to hold water against gravity so are still full. At this stage the soil is said to be at Field Capacity.

Plants access the ‘easy-to-get water’ first, but as the soil dries, capillary forces become too stronger and the plant has to work harder to draw the water from the soil. At a point termed Stress Point, plant growth is slowed and yield potential is lostreduced. Plants will survive beyond this point but become increasingly stressed. Stress point is variable because it is related to crop type, and rooting depth ands well as soil types.

At a certain pointlevel, plants can get no more water from the soil., At this point theyand become permanently wilted and die. This is the Permanent Wilting Point. Beyond the permanent wilting point there is still water in the soil but it is too tightly held for the plant to uptake (hydroscopic water).

Knowledge of field capacity (full point) stress point (trigger point) and permanent wilting point (death) are vital needed to successfully manage irrigation.

Available Water Holding Capacity (AWHC)

Beyond the permanent wilting point there is still water in the soil but it is too tightly held for the plant to uptake. The term ‘Available Water Holding Capacity’ (AWHC) refers to the water held between the field capacity and the permanent wilting point. This is the total plant available water.

Figure 5Figure 12 shows typical soil available water holding capacities (AWHC), field capacities (FC) and permanent wilting points (WP) for different soil textures in mm of water per 100mm of soil depth.

For the examples shown

Sands have low water storage (FC 12mm) but most of the water can be abstracted by the plant (WP 2mm) resulting in low AWHC = 10mm/100mm soil depth.

Loams have high water storage (FC 33mm) and most of the water can be abstracted by the plant (WP 13mm) resulting in high AWHC = 20mm/100mm soil depth.

Clays have high water storage (FC 38mm).However they retain much of the water and do not make it available to the plant (WP 24mm) resulting in moderate AWHC = 14mm/100mm soil depth.

Readily Available Water

The amount of water in a soil that supports maximum plant growth is known as ‘Readily Available Water’. It is the difference between Field Capacity and the Stress Point (Figure 6).

Insert visual diagram of terms from evaluation resources

As a rule of thumb only about half of the total amount of water is easily accessed by plants. The rest takes work to extract, and crop yield is decreased. So the default stress point is 50% of AWHC

Determining Available Water Holding Capacity

Determining The Available Water Holding Capacity determinesallows you to know the size of the soil’s water storage for from which plants to can draw water from.

This is aIt is the fundamental piece of information n needed to design and schedule irrigation. Table 1y gives an indication of Available Water Holding Capacity for the various soil classes Available Water Holding Capacilty.

T

At a base level to determine AWHC you need fourthree pieces of bites of information are required:

Class / Millimetres per 100 mm of soil depth
Down to 300 mm / Below 300 mm
Sand / 15 / 5
Loamy sand / 18 / 11
Sandy loam / 23 / 15
Fine sandy loam / 22 / 15
Silt loam / 22 / 15
Clay loam / 18 / 11
Clay / 17.5 / 11
Peat / 20-25 / >20-25

1.  A soil profile split into each horizon

1. The Ssoil texture of each horizonypes in the soil profile

3.  Information on WHC for each texture

1. The Ddepth of each horizons

5.  To obtain the information Information on WHC for the soil types

for 1, 2 and 4...... To get 1 and 2 – find a ruler, and and a spade, and dig a hole!

SFor 3 – soils WHC information for 3, can be obtained from regional council websites and Landcare research online data base “S-Map Online”. It can be determined on site using tools such as neutron probes.

As WHC is very variable, even within aone paddock, caution is needed when using the regional soil maps and databasesdatabases. Such information isThe information from maps and online databases is likely to be very general and at a coarse scale. S, and both soil properties and types should therefore be checked on site. If stones are present, the water holding capacity value should be reduced by the same percentage, i.e. if stones make up 30% of the soil volume, reduce the soil water holding capacity by 30%.

Soils specialists (pedologists) can give advice based on either knowledge of how certain soil types behave, or by direct measurement.

Detailed methods to determine WHC are outlined in the ‘extension resources’ section.

Soil Infiltration Rate

The soil infiltration rate is the speed at whichthat applied water, (rainfall or irrigation), can ienters absorbed by the soil. It is ddescribed as the millimetres depth of water infiltrated per hour (mm/hr).

Infiltration rates differ for eachaccording to soils typeproperties The rate at which this occurs is different for every soil and is and are also influenced by a number managementof practicesfactors. The key factors that influence soil infiltration are:

·  Texture -is the first influence with coarser texturessoils (sands and gravels) allow water to enter the soil absorbing water faster than finer ones (clays and silts)soils.

·  Management practices - these can affect the soils structural condition.; Ccultivation, stock and vehicle compaction can significantly reduce infiltration rates.

·  Soil Moisture – Infiltration Rrates also vary with soil moisture contentlevel, slowing as the soil becomes progressively wetter.

If water is applied faster than ithe soil can absorb it can enter the soil, ponding and/orwith associated run-off and or drainage will occur. Ponding also often results in preferential flow - drainage through the macro-pores . Applying irrigation in excess of the soil’s infiltration rateThis significantly reduces irrigation efficiency, is wasteful of the water resource and can cause crop damage.