Due to the manner in which water is applied by a drip irrigation system, only a portion of the soil surface and root zone of the total field is wetted unlike surface and sprinkler irrigation systems. Water flowing from the emitter is distributed in the soil by gravity and capillary forces creating the contour lines similar to onion shape. The exact shape of the wetted volume and moisture distribution will depend on the soil texture, initial soil moisture, and to some degree, on the rate of water application. Figs. 2.5 & 2.6 show the effects of changes in discharge on two different soil types, namely sand and clay. The water savings that can be made using drip irrigation are the reductions in deep percolation, surface runoff and evaporation from the soil. It is evident from the Fig.2.7 that soil moisture content in the soil always remains at or around the field capacity in drip irrigation, where as in sprinkler and surface irrigation methods, crops face over irrigation and water stress during certain period. In the line source type of drip irrigation system where the emitters are spaced very closely, individual onion patterns creates a continuous moisture zone. The knowledge about the wetting patterns under emitters is essential in selecting the appropriate
Fig. 2.5: Wetting patterns for sandy soils with high and low discharge rates emitters
Fig. 2.6: Wetting patterns for clay soils with high and low discharge rates emitters
14AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
Fig. 2.7: Moisture availability for crops in different irrigation methods
spacing of the emitters. Distance between emitters and emitter flow rates must match to the wetting characteristics of the soil and the amount and timing of water to be supplied to meet the crop needs.
Under drip irrigation, the ponding zone that develops around the emitter is strongly related to both the application rate and the soil properties.
The water application rate is one of the factors which determine the soil moisture regime around the emitter and the related root distribution and plant water uptake patterns. Drip irrigation systems generally consist of emitters that have discharge varying from 2.0 to 8.0 lph. In semi-arid climates, crop water use during the summer can be 6 to 8 mm/d with water supplied two or three times a week. When the water application exactly equal to the plant water need, then also, part of the water may not be used by the plant and it would most likely leach below the root zone. Therefore, lowering the emitter discharge to as close as possible to the plant water uptake rate can improve irrigation efficiency. Recently, microdrip irrigation systems have been developed that provide emitter discharges of 0.5 lph. These systems have been studied most intensively in greenhouses (Koenig, 1997), and preliminary results showed that they reduced water consumption of tomato plant by 38%, increased yield by 14 to 26%, and reduced leaching fraction by 10 to 40%. In a recent application on sweet corn under field conditions, Assouline et al. (2002) have shown that microdrip irrigation may improve yield, reduce drainage flux, and affect the water content distribution within the root zone, especially through an increased drying of the 0.60 to 0.90m soil layer compared with conventional drip irrigation.
The microdrip technology still raises some problems concerning the uniformity of application and the steadiness of the discharges. However, soil moisture regimes similar to those resulting from continual low water application rates can be achieved by means of pulsed drip irrigation. Infiltration experiments on a sandy loam soil showed that the water content distribution and the rate of wetting front advance under a pulsed water application were similar to water applied in a continuous manner, and those temporal fluctuations in flux and in soil water content exponentially damped with depth for periodic pulses
16 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
applied at the soil surface. Consequently, pulsed irrigation using conventional drip emitters could be one way of creating the water regime observed with continual low application rates while bypassing technical problems associated to microdrip emitters. The relationships between water application rates, soil properties, and the resulting water distribution for conventional emitters (2.0 lph) are well documented.
The wetting patterns during application generally consist of two zones:
(i) a saturated zone close to the emitter, and (ii) a zone where the water content decreases toward the wetting front. Increasing the emission rate generally results in an increase in the wetted soil diameter and a decrease in the wetted depth (Schwartzman and Zur, 1986; Ah Koon et al., 1990). In microdrip irrigation, field observations seem to indicate that there is no saturated zone and that the wetted soil volume is greater compared with that for conventional emitter discharges (Koenig, 1997). The relationship between the water application rate and the resulting water content distribution is complex because it is a three-dimensional outcome related to soil properties and crop uptake characteristics. Therefore, a quantitative representation of the flow processes by means of a simulation model could be beneficial in studying the effects of emitter discharge on the water regime of drip irrigated crops.
Many attempts have been made to determine water movement and wetting pattern under drip emitters using mathematical and numerical models. The Richards equation, formulated by Lorenzo A. Richards in 1931, describes the movement of water in unsaturated soils. It is a non-linear partial differential equation, which is often difficult to approximate. Partial differential equations are a type of differential equation which formulates a relation involving unknown functions of several independent variables and their partial derivatives with respect to those variables. Ordinary differential equations usually model dynamical systems whereas partial differential equations are used to model multi-dimensional systems. Darcy’s law was developed for saturated flow in porous media; to this Richards applied a continuity requirement and obtained a general partial differential equation describing water movement in unsaturated soils. The Richards’ equation
is based solely on Darcy’s law and the continuity equation. Therefore it is strongly physically based, generally applicable, and can be used for fundamental research and scenario analysis. The Richards equation can be stated in the following form:
( )
K = hydraulic conductivity, ψ = pressure head,
Z = elevation above a vertical datum, θ = water content, and
t
= time.Under drip irrigation, we have already discussed that only a portion of the horizontal and cross sectional area of the soil is wetted. The percentage wetted area as compared with the entire field covered with crops, depends on the volume and rate of discharge at each emitter, spacing of emitter and the type of soil being irrigated. For widely spaced crops, the percentage wetted area should be less than 67% in order to keep the area between the rows relatively dry for cultural practices.
Low value of percentage wetted area also reduces the loss of water due to evaporation and involves less cost. For closely spaced crops such as vegetables with rows and laterals spaced less than 1.8 m, percentage wetted area often approaches 100% (Keller and Bliesner, 1990). Several efforts have been made to estimate the dimensions of the wetted volume of soil under an emitter. Schwartzmass and Zurr (1985) assumed that wetted soil volume depends upon the hydraulic conductivity of the soil, discharge of the emitter and amount of water available in the soil. They developed the following empirical equations to estimate the wetted depth and width. The equations were derived using three-dimensional cylindrical flow geometry and results were verified from plane flow model.
18 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
...(2.3)
By combining the above two equations, we can find out the relationship between depth of wetting front, Z and width of wetted soil volume (w).
The relationship can be expressed as follows.
w = 0.0094 (Z)0.35 q0.33 K-0.33 ...…...(2.4) where
Z = depth of wetting front, m
w = wetted width or diameter of wetted soil, m Vw = volume of water applied, l
K = saturated hydraulic conductivity of soil, m/s q = discharge of emitter, lph
Example 2.1.
In a banana orchard, emitters of 4 lph discharge capacity are operating.
The soil is sandy loam and rooting depth is 1.2 m. Saturated hydraulic conductivity of the soil is 30 mm/h. Find the width of the wetted soil volume.
Solution. It is given:
q = 4 lph
Rooting depth is 1.2 m. It will be taken as vertical depth of wetting front. So it is Z.
K = 30 mm/h = 8.33 x 10-6m/s
The equation for width of wetted soil volume is
20 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
Mohammed (2010) developed a simple empirical model to determine the wetting pattern geometry from surface point source drip irrigation system. The wetted soil volume was assumed to depend on the saturated hydraulic conductivity, volume of water applied, average change of moisture content and the emitter application rate. The following assumptions were made.
• A single surface point source irrigated a bare soil with a constant discharge rate.
• The soil is homogeneous and isotropic.
• No water table present in the vicinity of root zone.
• The evaporation losses are negligible.
• The effect of soil properties is represented by its porosity and saturated hydraulic conductivity.
• The value of porosity equals the value of saturated moisture content. It could be obtained using an equation given by Hillel, (1982) which states:
n = porosity of the soil
θs = Moisture content at 0 bars
ρb = bulk density of the soil (measured) ρp = particle density of the soil
It was considered that wetted radius and wetted depth of soil volume depends upon certain variables. The functional relationship among all the variables can be defined as follows:
r ∝ f1 (K, n, qw, Vw)...…...(2.7) z ∝ f2 (K, n, qw , Vw)...…...(2.8) where,
r = wetted radius
K = soil hydraulic conductivity n = soil porosity
qw = application rate
Vw = volume of water applied z = depth of wetted zone.
If we consider the equation given by Hillel (1982), the above two equations can be written as
r ∝ f1 (K, θS , qw , Vw)...…... (2.9) z ∝ f2 (K, θS , qw , Vw)...…... (2.10) Ben-Asher et al. (1986) investigated the infiltration from a point drip source in the presence of water extraction using an approximate hemispherical model. For infiltration from a point source without water extraction, they established the following:
2
θ
Sθ
≈∆ …... (2.11)
The new variable ∆
θ
is called the average change of soil moisture content. This leads to:r ∝ f1 (K, ∆θ , qw , Vw)...…... (2.12) z ∝ f2 (K, ∆θ , qw , Vw)...…... (2.13) According to the approaches introduced by Shwartzman and Zur (1986) and Ben Asher et al. (1986), the nonlinear expressions describing wetting pattern may take the general forms as:
r = ∆θα Vwβ qwγ Kλ ...…... (2.14) z = ∆θρ Vwσ qwδ Kς…... (2.15)
22 AN INTRODUCTION TO DRIP IRRIGATION SYSTEM
Once they identified the model structure and order, the coefficients were estimated in some manner. To determine the coefficients of Eq.
2.14 and 2.15, four available published experimental data of Taghavi et al. (1984), Anglelakis et al. (1993), Hammami et al. (2002), and Li et al. (2003) were adopted. The choice of these experiments was essentially based on availability of their convenient data. A nonlinear regression approach was used to find the best-fit parameters for the Eq. 2.14 and 2.15. The following equations are obtained:
r = ∆θ -0.5626Vw0.2686 qw-0.0028 K-0.0344…... (2.16) z = ∆θ-0.383 Vw0.365 qw-0.101 K 0.1954…... (2.17) where, r and z (cm) are consistent units used in this approximations, Vw (ml), qw (ml/h), and K in (cm/h).
Cook et al. (2006) developed a model and implemented in the WetUp software which uses data on the approximate radial and vertical wetting distances for different soils and discharge rates estimated by using analytical methods. WetUp is an easy to use and freely available software tool (http://www.clw.csiro.au/products/wetup), which will definitely help to graduate students to fine tune their research work on crop water management under drip irrigation systems. The program is a results of collaborative efforts among the Commonwealth Scientific and Industrial Research Organization (CSIRO), Cooperative Research Centre (CRC) for Sustainable Sugar Production and the National Program for Irrigation Research and Development (NPIRD) in Australia and the methods described by Thorburn et al. (2002). You can not provide the manual inputs but can always select all the required inputs from pre-defined selection boxes and drop down menus. Every simulation window opens with predefined values and the user can easily adjust or select the soil type, emitter flow rate, the maximum time and whether a surface or buried emitter should be simulated starting under dry, moist or wet soil condition. Different soils can be chosen by double clicking on the ‘Select Soil Type’ section on the simulation windows, or by choosing an appropriate button in the button bar. There are currently 29 soils which are based on average soil properties published by Clapp and Hornberger (1978) and measured field soils from
Queensland in Australia published by Verburg et al. (2001). Flow rates may be selected in the range of 0.5 to 2.7 l/hr for an irrigation time of 1 to 24 hours. The depth of buried drip lines may be changed in the range of 0.1 to 1.5 m. In Indian conditions, we normally use 4 to 8 lph emitters. Since only 29 soils have been included, you may not find your soils and hence this is a limitation. However, suppose you want to simulate wetting patter of 4 lph emitter after 12 hrs, then you can think of using emitter of 2 lph and simulate it for 24 hrs. This software is meant as an educational tool and you can at least see the effect of changing the variables on wetting patterns.