**4.4 Distribution variability of effective rainfall**

With global warming and climate change, greater competition is expected among water users, and paddy irrigation may be sacrificed during water shortage in dry months favouring domestic and industrial users. However, rice granaries practicing multiple cropping have yet to improve on the use of "effective rainfall". Currently, the measurement of rain falling in a rice growing area is based solely on the available rain gauge network. These gauges are located at convenient locations which may not be representative of the whole rice growing

Paddy Water Management for Precision Farming of Rice 137

15 16 17

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Fig. 17. (continues on next page) The radar derived rainfall data from 17 Virtual Rainfall Stations in a 2300 ha Sawah Sempadan Irrigation Compartment produced rainfall

distribution pattern which otherwise would always be uniformly distributed since there is

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**Legend**

4

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**Rainfall 16 Mar**

only one rain-gauge for the whole area

3

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 3

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Virtual Rainfall Stn. Tanjung karang Lots

area. Hence, under- or over-estimation of rainfall distribution and runoff occurs and consequently affects the management of floods during rainy seasons or base flow for irrigation during dry seasons. Therefore, better estimates of mean areal rainfall are needed as contribution of effective rainfall in the water balance during the irrigation season.

Fig. 16. Description of Different Water Efficient Regimes (Mao Zhi, 2000)

A new technique to improve rainfall distribution estimation based on weather radar-derived rainfall throughout the rice growing area was developed by UPM using GIS tools. Virtual rainfall stations are created uniformly throughout the area to improve the spatial distribution of rainfall over a rice granary or a watershed with low density rain gauge network. Virtual rainfall stations can be distributed in terms of grid centres to cover the whole study area as shown in Fig. 17. The rainfall data for these virtual rain gauges are estimated from raw radar data available from the Malaysian Meteorological Department using a newly developed Program called UPM ViRaS RaDeR ver1.0 (Amin et al., 2010). The derived rainfall data is

area. Hence, under- or over-estimation of rainfall distribution and runoff occurs and consequently affects the management of floods during rainy seasons or base flow for irrigation during dry seasons. Therefore, better estimates of mean areal rainfall are needed

as contribution of effective rainfall in the water balance during the irrigation season.

Fig. 16. Description of Different Water Efficient Regimes (Mao Zhi, 2000)

A new technique to improve rainfall distribution estimation based on weather radar-derived rainfall throughout the rice growing area was developed by UPM using GIS tools. Virtual rainfall stations are created uniformly throughout the area to improve the spatial distribution of rainfall over a rice granary or a watershed with low density rain gauge network. Virtual rainfall stations can be distributed in terms of grid centres to cover the whole study area as shown in Fig. 17. The rainfall data for these virtual rain gauges are estimated from raw radar data available from the Malaysian Meteorological Department using a newly developed Program called UPM ViRaS RaDeR ver1.0 (Amin et al., 2010). The derived rainfall data is

Fig. 17. (continues on next page) The radar derived rainfall data from 17 Virtual Rainfall Stations in a 2300 ha Sawah Sempadan Irrigation Compartment produced rainfall distribution pattern which otherwise would always be uniformly distributed since there is only one rain-gauge for the whole area

Paddy Water Management for Precision Farming of Rice 139

then compared and calibrated with actual gauge rainfall records for the same periods to identify the calibration factor. RaDeR provides rainfall distribution pattern which otherwise would always be uniformly distributed since there is only one rain-gauge for the whole area. The calibrated radar derived rainfall data will next be used as improved rainfall input in the hydrological model for watershed runoff estimation. On the other hand, knowing the amount of rainfall that occurred in a rice granary or a farm, a suitable amount of irrigation water can be supplied precisely and better irrigation water management can be adopted. Irrigation can be stopped when enough rain water has already refilled the soil moisture reservoir or standing water depth in the paddy fields. Hence the effective rainfall will save

In anticipation of future greater competition for irrigation water due to climate change and global warming, paddy water management should be more focused towards water saving and precision irrigation. This book chapter has described new indicators for evaluating the performance of different aspects of an irrigation system for rice cultivation. A GIS-based interactive assessment tool is given using a new concept to characterize the irrigation delivery performance as the season advances. The weakness of a widely used Relative Water Supply (RWS) concept is overcome by using the new indicators, viz. Rice Relative Water Supply (RRWS), Cumulative Rice Relative Water Supply (CRRWS) and Ponding Water Index (PWI). The RRWS can distinctly characterize the oversupply condition for RRWS > 1.0 and undersupply condition for RRWS < 1.0 on irrigation delivery for any given period. A value of 1.0 for RRWS indicates an irrigation delivery that perfectly matches the actual field water demand. A user-interface was developed for structuring the assessment tool within ArcGIS platform. The system can instantly give information on the uniformity of water distribution and the shortfall or excess and what decisions to adopt for the next period. The results are displayed on the computer screen together with colour-coded maps, graphs and tables in a comprehensible form. The system can be adopted as an analytical and operational tool for the irrigation managers to evaluate various water allocation scenarios

Water savings can be obtained by practicing precision farming of rice in lowland paddy fields. However a rapid assessment of the paddy soil variability needs to be determined, for example through mapping of the bulk electrical conductivity (ECa) of the paddy fields, so that variable treatments of the management zones can be adopted to save the precious resources. ET monitoring is necessary to determine the required amount of water at each crop growth stage, and the rainfall distribution pattern in the irrigation scheme should be considered to make better use of effective rainfall with respect to the stage of crop development. The practice of precision farming (i.e. applying the right input, at the right place, at the right time, at the right amount and in the right manner using the right tools) will ensure high water and land productivity for a sustainable rice production to feed the

The financial support for the research provided by the Government of Malaysia through MOSTI is gratefully acknowledged. Cooperation from all members of the Precision Farming

some amount of irrigation water supply and used for other purposes.

**5. Summary** 

and water management options.

growing world population.

**6. Acknowledgements** 

Fig. 17. (continued) The radar derived rainfall data from 17 Virtual Rainfall Stations in a 2300 ha Sawah Sempadan Irrigation Compartment produced rainfall distribution pattern which otherwise would always be uniformly distributed since there is only one rain-gauge for the whole area

then compared and calibrated with actual gauge rainfall records for the same periods to identify the calibration factor. RaDeR provides rainfall distribution pattern which otherwise would always be uniformly distributed since there is only one rain-gauge for the whole area. The calibrated radar derived rainfall data will next be used as improved rainfall input in the hydrological model for watershed runoff estimation. On the other hand, knowing the amount of rainfall that occurred in a rice granary or a farm, a suitable amount of irrigation water can be supplied precisely and better irrigation water management can be adopted. Irrigation can be stopped when enough rain water has already refilled the soil moisture reservoir or standing water depth in the paddy fields. Hence the effective rainfall will save some amount of irrigation water supply and used for other purposes.
