**2.4.2 Normal irrigation requirements**

The required irrigation water during the normal irrigation period shall be allocated on the basis of the equation (2):

$$\text{GIR}\_{\text{j}} = \frac{\text{(ET}\_{\text{O}}\text{)}\_{\text{j}} \times \text{K}\_{\text{c}} + \text{SP}\_{\text{j}} \text{-ER}\_{\text{j}}}{\text{IE}} \tag{2}$$

where,

GIRj = gross irrigation water requirement (mm/day) (ET0)j = reference crop evapotranspiration (mm/day)

relevant government agencies such as the Tanjung Karang Rice Irrigation Scheme Authority (IADA) for different ISAs, the Department of Irrigation and Drainage (DID), Department of Agriculture (DOA), Department of Survey and Mapping Malaysia (JUPEM) and Malaysia Meteorological Department (MMD). The detailed configuration of the irrigation canals, irrigation head regulator, Constant Head Orifice (CHO) offtake structures and specifications, stage and discharge data for the main canal were obtained from the Irrigation and Drainage Authority of the Scheme and also from the DID Headquarters, Malaysia. Database development is the crucial task to bring all the information obtained into a GIS database. All

Water demand estimation is the primary considerations for planning, design and evaluating of the irrigation scheduling of a scheme. In Malaysia, the recommended design presaturation and supplementary irrigation requirements for the rice irrigation systems are 2.31 l/s/ha (20 mm/day) and 1.16 l/s/ha (10 mm/day), respectively. The total water requirement for rice production is about 1000–1500 mm depending on characteristics of the schemes. A quantitative estimation of the major components of field water balance provides management decisions on how the scheme ought to be operated to ensure better

A huge amount of water is consumed to inundate fields for presaturation before planting of the crop. The water required during presaturation period can be determined as follows:

LS S IR +EP +SP+SW SAT

The required irrigation water during the normal irrigation period shall be allocated on the

×

( O c ) j j <sup>j</sup>

IE

ET K +SP -ER

*IE* <sup>=</sup> (1)

= (2)

the data were properly registered and assembled in GIS platform.

distribution of irrigation water and the delivery performance.

SAT = water requirement during presaturation period (mm/day)

SW = additional supply to maintain the initial depth of flooding (mm/day)

j

GIR

GIRj = gross irrigation water requirement (mm/day) (ET0)j = reference crop evapotranspiration (mm/day)

IRLS = water requirement to saturate the soil (mm/day) EPs = evaporation loss from saturated soil surface (mm/day)

SP = seepage and percolation losses (mm/day)

IE = overall irrigation efficiency

basis of the equation (2):

**2.4.2 Normal irrigation requirements** 

**2.4.1 Presaturation irrigation requirements** 

**2.4 Water demand estimation** 

Where,

where,

SPj = seepage-percolation loss (mm/day) ERj = effective rainfall (mm/day) Kc = crop coefficient IE = overall irrigation efficiency, which is assumed to be 45% ,[14].

For presaturation water depth, the DID recommendation of 20 mm/day is used. This would help to maintain the standing water depth of 100 mm for the normal irrigation period. The minimum standing water depth (SWmin) is maintained at 50 mm. The seepage-percolation (SP) rate of 2-3 mm/day is considered throughout the growth period [14 and 15].

If part of the water requirement is met by utilization of rainfall during crop growing period, then the net irrigation requirement on a particular day is determined as:

$$\text{NIR}\_{\text{j}} = \text{ET}\_{\text{j}} + \text{SP}\_{\text{j}} \text{--ER}\_{\text{j}} + \text{SW}\_{\text{j}} \text{--SW}\_{\text{j}-1} \tag{3}$$

where, ETj is EToj \* kc, SWj is the required standing water depth for a particular day, and SWj-1 is field water level at the beginning of irrigation supply on (j-1)-th day. The NIRj is determined as in Equation 3 when paddy fields remain in the condition SWj ≥ SWj-1. However, this condition is rare during peak water demand and it is possible only by storing a significant amount of rainfall in the paddy fields. The inequality between SWj and SWj-1 leads to different water balance scenarios as well as water allocation rules, which are determined mainly by SWj-1 that falls short or exceeds the required standing water depth. The conditions for the estimation of the net irrigation requirements are summarized in Table 1.


Table 1. Net Irrigation Requirements for Different Water Balance Scenarios

### **2.5 Assessment of the irrigation delivery performance for rice**

Indicators and measures of irrigation water delivery performance are best when those can be used to evaluate the irrigation delivery performance and as management tool to keep track of the water delivery performance as the season progresses. In this regards, the RWS concept is appropriate and can be applied for paddy rice and upland rice or other crops. This discussion however is restricted mainly to paddy rice for characterizing the irrigation delivery performance using the RWS concept.

Paddy Water Management for Precision Farming of Rice 115

gives incorrect determination to characterize an oversupply condition on irrigation

The Rice Relative Water Supply (RRWSj) is defined as the ratio of the total supply as Irrigation requirement (IRj) and Effective Rainfall (ERj) to the total demand as the sum of the difference between Maximum Ponding Water Depth and Present Ponding Water Depth (WSmaxj-WSj) for a particular irrigation period; Evapotranspiration (ETj) and Seepage-Percolation (SPj) in the service areas for a duration being considered. It can distinctly characterize the oversupply for RRWSj > 1.0 and undersupply for RRWSj < 1.0 for any given period as the season advances. The value of RRWSj = 1.0 indicates irrigation supply is perfectly matched with the field water demand. Incorporating depleted ponding water (WSmaxj - WSj) into eq. (6) is the modification for the RWS concept given by [6] especially useful for evaluating irrigation delivery in rice-based systems. The RRWS is expressed as

( )

The oversupply and undersupply can be simply identified for any given irrigation period with the actual RRWS value compared to the RRWS = 1.0. For a particular period, irrigation supply is gradually increased with the amount of depleted ponding water until it reaches the maximum level in the field. Without considering this amount, RWS gives higher values, which are normally characterized as the false oversupply condition. In fact, it is not necessarily an oversupply. A value of RWS = 0.8 may not represent a problem, rather it may provide an indication that farmers are practicing deficit irrigation supply to maximize returns on water [16]. This remark can be adopted for operating irrigation system even at RRWS = 0.5 for a particular period to overcome water shortage and could be helpful to store

The Cumulative Rice Relative Water Supply (CRRWS) is defined as the accumulated value of RRWS, which is the ratio of supply to the demand computed over short intervals of time starting from a particular time of the season. The advantage of CRRWS is similar throughout the season like CRWS. In addition, CRRWS can overcome the weakness of RWS and CRWS.

> ( ) <sup>n</sup> j j

The slope and the trend of the CRRWS concept provide useful management inferences simpler than CRWS. The values of CRRWS for daily, weekly or any other short interval of cropping season with time interval can be plotted along the x-axis and CRRWS value along the Y-axis. This plot carries with the curve designated as CRRWS = 1.0. If computed CRRWS line follows the CRRWS = 1.0 line, it means that irrigation deliveries are entirely matched

j 1 j j jj

<sup>=</sup> WSmax WS +ET SP <sup>+</sup> <sup>=</sup> − +

IR ER

∑ (7)

j

RRWS

**2.5.3 Cumulative Rice Relative Water Supply (CRRWS)** 

j

CRRWS

It can be defined for a particular period as follows:

j j

IR ER

WS max WS +ET SP <sup>+</sup> <sup>=</sup> − +

j j jj

(6)

deliveries for not considering this additional water supply for rice production.

**2.5.2 Rice Relative Water Supply (RRWS)** 

more rainfall if WSmaxj is retained.

follows:

### **2.5.1 Relative water supply (RWS) concept**

The available water supply and the water demand are the two most crucial factors for planning, design and operation of any irrigation system. The ratio of supply and demand constitutes an important concept called the Relative Water Supply [6]. This concept is actually the inverse of the engineering irrigation efficiency, output over input. The irrigation supply is the supply measured at the point of interest. The total water supply is defined to include both the irrigation supply and the effective rainfall during the period being considered. The effective rainfall is the fraction of the total rainfall over the irrigation command area that potentially could substitute for the irrigation supply. The total water demand is considered from losses due to crop evapotranspiration and seepage-percolation for the same duration. The RWS is mathematically expressed as follows:

For land preparation period or pre-saturation period,

$$\text{RWS}\_{\text{j}} = \frac{\text{IR}\_{\text{j}} + \text{ER}\_{\text{j}}}{\text{EP}\_{\text{j}} + \text{SP}\_{\text{j}} + \text{LS}\_{\text{j}}} \tag{4}$$

For normal crop growth period,

$$\text{RWS}\_{\text{j}} = \left(\frac{\text{IR}\_{\text{j}} + \text{ERj}}{\text{ETj} + \text{SPj}}\right) \tag{5}$$

where, LSj = Land soaking water requirement in cm. The lower bound of RWS = 1.0 and the higher bound of RWS = 1.15 or more considered as the management strategy to maintain adequate water supply [7]. For any period, the value less than 1.0 represents undersupply and the value more than the upper bound represents oversupply. In fact, the upper bound value is not standard for characterizing the irrigation delivery as it depends on many factors involved in the irrigation systems. The RWS value can be maintained at lower bound level considering the expected stochastic rainfall in the next irrigation period. If no rainfall occurred, the paddy fields will remain undersupplied.

The RWS concept has proven to be a useful tool for understanding the performance of the irrigation systems and the impact on performance of the behavior of the major participants (the irrigation engineers and the farmers) in the irrigation process. It is useful for analysis and interpretation of irrigation performance for different time intervals and for different locations at system and sub-system levels. The computation flexibility in terms of time and space of the RWS makes it easy to use for evaluating the irrigation delivery performance. In addition to the potential for evaluating irrigation performance, the RWS concept is useful to evaluate the relative equity of water service with a system. It has also proven useful in understanding the decision rules that characterize system operation.

Traditionally rice is grown under continuous submergence or intermittent or variable ponding conditions depending on the farmer's choice and also on the water resources. This is the basic difference for the rice irrigation management system from other crops. Some additional water is required to maintain the standing water depth in the field due to the difference between the Desired or Maximum Standing Water Depth (WSmaxj) and the Present Standing Water Depth (WSj). The widely used Relative Water Supply (RWS) concept gives incorrect determination to characterize an oversupply condition on irrigation deliveries for not considering this additional water supply for rice production.

### **2.5.2 Rice Relative Water Supply (RRWS)**

114 Current Issues of Water Management

The available water supply and the water demand are the two most crucial factors for planning, design and operation of any irrigation system. The ratio of supply and demand constitutes an important concept called the Relative Water Supply [6]. This concept is actually the inverse of the engineering irrigation efficiency, output over input. The irrigation supply is the supply measured at the point of interest. The total water supply is defined to include both the irrigation supply and the effective rainfall during the period being considered. The effective rainfall is the fraction of the total rainfall over the irrigation command area that potentially could substitute for the irrigation supply. The total water demand is considered from losses due to crop evapotranspiration and seepage-percolation

j j

<sup>+</sup> <sup>=</sup> + + (4)

(5)

IR ER

EP SP LS

jj j

j j <sup>j</sup> j j IR ER

ET SP ⎛ ⎞ <sup>+</sup> =⎜ ⎟ + ⎝ ⎠

where, LSj = Land soaking water requirement in cm. The lower bound of RWS = 1.0 and the higher bound of RWS = 1.15 or more considered as the management strategy to maintain adequate water supply [7]. For any period, the value less than 1.0 represents undersupply and the value more than the upper bound represents oversupply. In fact, the upper bound value is not standard for characterizing the irrigation delivery as it depends on many factors involved in the irrigation systems. The RWS value can be maintained at lower bound level considering the expected stochastic rainfall in the next irrigation period. If no rainfall

The RWS concept has proven to be a useful tool for understanding the performance of the irrigation systems and the impact on performance of the behavior of the major participants (the irrigation engineers and the farmers) in the irrigation process. It is useful for analysis and interpretation of irrigation performance for different time intervals and for different locations at system and sub-system levels. The computation flexibility in terms of time and space of the RWS makes it easy to use for evaluating the irrigation delivery performance. In addition to the potential for evaluating irrigation performance, the RWS concept is useful to evaluate the relative equity of water service with a system. It has also proven useful in

Traditionally rice is grown under continuous submergence or intermittent or variable ponding conditions depending on the farmer's choice and also on the water resources. This is the basic difference for the rice irrigation management system from other crops. Some additional water is required to maintain the standing water depth in the field due to the difference between the Desired or Maximum Standing Water Depth (WSmaxj) and the Present Standing Water Depth (WSj). The widely used Relative Water Supply (RWS) concept

for the same duration. The RWS is mathematically expressed as follows:

j

RWS

RWS

For land preparation period or pre-saturation period,

occurred, the paddy fields will remain undersupplied.

understanding the decision rules that characterize system operation.

For normal crop growth period,

**2.5.1 Relative water supply (RWS) concept** 

The Rice Relative Water Supply (RRWSj) is defined as the ratio of the total supply as Irrigation requirement (IRj) and Effective Rainfall (ERj) to the total demand as the sum of the difference between Maximum Ponding Water Depth and Present Ponding Water Depth (WSmaxj-WSj) for a particular irrigation period; Evapotranspiration (ETj) and Seepage-Percolation (SPj) in the service areas for a duration being considered. It can distinctly characterize the oversupply for RRWSj > 1.0 and undersupply for RRWSj < 1.0 for any given period as the season advances. The value of RRWSj = 1.0 indicates irrigation supply is perfectly matched with the field water demand. Incorporating depleted ponding water (WSmaxj - WSj) into eq. (6) is the modification for the RWS concept given by [6] especially useful for evaluating irrigation delivery in rice-based systems. The RRWS is expressed as follows:

$$\text{RRWS}\_{\text{j}} = \frac{\text{IR}\_{\text{j}} + \text{ER}\_{\text{j}}}{\left(\text{WS}\,\text{max}\_{\text{j}} - \text{WS}\_{\text{j}}\right) + \text{ET}\_{\text{j}} + \text{SP}\_{\text{j}}} \tag{6}$$

The oversupply and undersupply can be simply identified for any given irrigation period with the actual RRWS value compared to the RRWS = 1.0. For a particular period, irrigation supply is gradually increased with the amount of depleted ponding water until it reaches the maximum level in the field. Without considering this amount, RWS gives higher values, which are normally characterized as the false oversupply condition. In fact, it is not necessarily an oversupply. A value of RWS = 0.8 may not represent a problem, rather it may provide an indication that farmers are practicing deficit irrigation supply to maximize returns on water [16]. This remark can be adopted for operating irrigation system even at RRWS = 0.5 for a particular period to overcome water shortage and could be helpful to store more rainfall if WSmaxj is retained.

### **2.5.3 Cumulative Rice Relative Water Supply (CRRWS)**

The Cumulative Rice Relative Water Supply (CRRWS) is defined as the accumulated value of RRWS, which is the ratio of supply to the demand computed over short intervals of time starting from a particular time of the season. The advantage of CRRWS is similar throughout the season like CRWS. In addition, CRRWS can overcome the weakness of RWS and CRWS. It can be defined for a particular period as follows:

$$\text{CRRWS}\_{\text{j}} = \sum\_{j=1}^{n} \frac{\text{IR}\_{\text{j}} + \text{ER}\_{\text{j}}}{(\text{WS}\,\text{max}\_{\text{j}} - \text{WS}\_{\text{j}}) + \text{ET}\_{\text{j}} + \text{SP}\_{\text{j}}} \tag{7}$$

The slope and the trend of the CRRWS concept provide useful management inferences simpler than CRWS. The values of CRRWS for daily, weekly or any other short interval of cropping season with time interval can be plotted along the x-axis and CRRWS value along the Y-axis. This plot carries with the curve designated as CRRWS = 1.0. If computed CRRWS line follows the CRRWS = 1.0 line, it means that irrigation deliveries are entirely matched

Paddy Water Management for Precision Farming of Rice 117

evaluate the irrigation delivery performance using the selected performance indictors as the

Fig. 2. The RIMIS Menu for the Tanjung Karang Rice Irrigation Scheme (TAKRIS) in ArcGIS

The user needs to enter the required inputs and choose the appropriate command button to view the results. Inputs can be directly fed into the dialog window by clicking on the command button "Data from Input Files" or manually (Fig. 3). All required inputs stored in Input Files are instantly retrieved into TextBoxes by each irrigation service area i.e. "ISAI: Input Daily Information (I)". Command Buttons for new irrigation delivery performance indicators, Rice Relative Water Supply (RRWS), Cumulative Rice Relative Water Supply (CRRWS) and Ponding Water Index (PWI) together with the Relative Water Supply (RWS) concept are shown in Fig. 3. Users may feed accurate information from other sources manually if available and reliable. To compute the performance indicators correctly, users should enter the actual information of rainfall and reference crop evapotranspiration on the day as they are available at the end of the irrigation day. A sub-routine was developed to compute the daily potential crop evapotranspiration as shown in Figures 4a and 4b. This sub-module allows an irrigation manager to compute the daily Reference Crop Evapotranspiration (ETo) using the Internationally recommended FAO Penman-Montieth Equation based on the available climatic information for a particular day. However studies by Hassan (2006) show that ETo calculated by the Penman-Montieth Equation gives an underestimation of ETo by 10% compared to ETo from micro-lysimeters installed in the

season advances.

paddy fields.

with the field water demand for a particular irrigation period. An increasing slope of the actual CRRWS curve with CRRWS = 1.0 means that irrigation supply can be slightly curtailed in the next period. On the other hand, if the slope is downwards, supply has to be increased.
