1. Introduction
Rice is the main food crop in Malaysia. Rice production of the country has achieved 72% self-sufficiency level (SSL) with an annual production of 3.5 million tonnes a year [
1]. The Agriculture and Agro-based Industry Ministry targets the country to achieve a 100 percent self-sufficiency level (SSL) in paddy production by 2020. Irrigation is crucial to the world’s food grain production, because 40% of all crops and close to 60% of cereal production comes from irrigated agriculture [
2], even though irrigated lands comprise only 20% of the arable land [
3]. In Asia, irrigated agriculture uses 90% of the total freshwater, and more than half of this irrigates rice. About 75% of the global rice volume is produced in the irrigated low lands (Cantrell 2004). There are an estimated 150 million hectares of rice lands worldwide, 50% of which are irrigated, usually with continuous flooding for most of the crop season [
4]. In many irrigated areas, rice is grown as a monoculture with two rice crops every year. Global water and food security are two of the most important challenges in the 21st century to supply sufficient food for the increasing population while sustaining a stressed environment threatened by climate change.
Water management is a difficult task for a large rice irrigation system. Different sub-systems, such as soil, water, climate, nutrients, plant, management systems, and their complex dynamics work in the paddy field environment. Furthermore, an individual irrigation scheme has its physical and unique characteristics. The effects of climate change are significant on water demand for irrigation that is continuously being aggravated by unsustainable practices like over-use of chemical fertilizers and poor water management. Excessive irrigation deliveries generate a huge amount of return flows containing fertilizers, insecticides, and pesticides from paddy fields in Malaysia. Eventually, drainage water from paddy fields loses essential agrochemicals and pollutes surface water resources. Poor and uneven water distributions were often criticized as the major bottleneck in attaining efficient water use in rice irrigation systems in Malaysia [
5,
6].
Deep percolation in water-intensive paddy rice crop field is a major outflow and needs due attention. The rate of deep percolation depends on soil type, puddling intensity, hydraulic conductivity, depth of ponding, etc. It is now well known that the water policies need to facilitate market-based approaches to water allocation and commercialization of agriculture. Only a fraction of irrigation water applied to the fields is utilized by the plants. Some portion of the applied water that is not consumed in agricultural fields flows to streams/drainage canals or is percolated downwards. The movement of water horizontally into the bunds and then vertically downwards to groundwater through the undisturbed soil column within the bunds is termed lateral percolation [
7]. Bhuiyan et al. [
8] reported that seepage and percolation (S & P) are site-specific and depend on soil texture, water table depth, proximity to drainage outlet, and farmer’s field water management status. Ghani [
9] reported that in addition to the above factors, seepage and percolation at the field level are affected by puddling and the standing water depth status of the rice fields and the crop growth stages.
With a rising irrigation water requirement and developing competition all around water utilizing areas, the world now faces challenges to convey a great deal more food with less water. This objective will be sensible only if appropriate methodologies are found to get water savings and additionally more effective water uses in agriculture. Rosenzweig et al. [
10] assessed alterations in crop water requirement and water availability to determine the reliability of the irrigation system. Over the last few decades, in conjunction with fast population growth and commercial concentration in urban centres, which are affected by financial growth, power shortages have grown to be an issue, particularly in seaside areas, and water shortages have taken place in northern China, mainly in the Yellow River Basin [
11].
Quantification of the amount of water used is very crucial for understanding and finding water use efficiency at an irrigation system level. Irrigation return flow consists of surface and subsurface flows. Water balance models, considering both components, can predict the return flow for re-use in paddy fields [
12]. Additionally, a field scale investigation of water flow in paddy rice fields involves the interaction of very complex processes, which is, relatively speaking, very difficult, costly, and time consuming. Therefore, numerical modeling is a fast and inexpensive approach with which to study water movement and optimal irrigation management practices. Hydrus-1D [
13] is a numerical model that has been widely tested by many researchers to predict water flow in paddy fields under different irrigation and management practices [
2,
14,
15,
16,
17]. However, no work has been done yet to check the accuracy of this model for simulating water flow in broadcasted and transplanted paddy rice fields in Tanjung Karang Rice Irrigation Scheme (TAKRIS), which leads researchers to question the usefulness of this model.
Modeling of water flow in rice field becomes a challenge, as rice is a highly water demanding crop; thus, it poses a greater risk of water loss in both surface and subsurface waters [
12]. In addition, rice is a shallow-rooted crop and the domain of the root zone is about 30–40 cm below the soil surface, which can lead to considerable water loss if it leaches under irrigated or high rainfall conditions [
18]. As the water movement in a rice field is vertical due to constant ponding water condition, the one-dimensional model can be used effectively [
12]. Furthermore, since the seepage in paddy plots is minimized, and water and solute transport can be simplified to a vertical movement [
19], thus, the Hydrus-1D model can be used in the present study, even though it has been often used by many researchers to simulate water flow and solute transport in flooded rice fields [
18,
20]. Indeed, no study is reported yet on this important aspect in Malaysia. Mostly previous studies were based on large-scale estimation of water balance components using multifarious parameters that may not reflect the true condition of paddy fields [
21,
22,
23]. In order to overcome these challenges, this study was carried out to evaluate and model the water movements and losses through the surface and sub-surface water leaving from a paddy field for better management practices through intensive field observations using modern monitoring devices together with sensors, and data logging and analysis techniques. In most irrigation projects, like any other countries, in Malaysia’s agricultural fields, in particular in its paddy rice fields, a huge amount of valuable irrigation water is lost through different processes from rice fields that needs to be quantified to determine the actual water balance component. This study, therefore, wishes to investigate the water movement in flooded paddy rice fields during two consecutive rice-growing seasons and then evaluate it using Hydrus-1D numerical model. We do not only evaluate water losses via subsurface water but also via an intensive investigation of the water balance component and productivity analysis, which were conducted to estimate water losses through surface and subsurface water using modern monitoring devices together with sensors and data logging and analysis techniques.
4. Discussion
The total amount of water use (irrigation + rainfall) was 116.5 cm for off-season and 90.4 cm for main season. Of this, 60% to 77% was applied by irrigation during two rice growing seasons, respectively. The mean values of ET were 0.52 and 0.56 cm day
−1 for both seasons, respectively. During both seasons, we revealed that ET was low at early stages and then started to increase gradually as rice plant growing into reproductive stage. Globally, estimates of rice evapotranspiration range from 45 to 70 cm season
−1, depending on the climate and growing season [
35]. In South and Southeast Asia, ET ranges from 0.4 to < 1 cm day
−1 [
36]. Thus, in the current study, the total amount of measured ET was 48.5 cm for the off-season and 55.2 cm for main season, respectively. However, the observed values of ET in the present study were within 0.28–0.71 cm day
−1, which are usually quoted values for major rice producing areas in Asia [
5]. Abbdullah et al. [
21] estimated crop evapotranspiration using micro-paddy lysimeter under same plot and reported ET value of 0.3 to 0.7 cm day
−1. Rowshon et al. [
22] estimated crop evapotranspiration using Marriott tube lysimeter and observed ET value of 0.4–0.9 cm day
−1. Lage et al [
37] conducted lysimeter experiment and reported daily average rice evapotranspiration rate of 0.67 cm day
−1. According to [
38], typical evapotranspiration values of rice fields are 0.4 to 0.5 cm day
−1 for wet season and 0.6 to 0.7 cm day
−1 but can be as high as 1 to 1.1 cm day
−1 in subtropical regions. Several researchers reported 0.4 to 0.9 cm day
−1 [
5,
22,
39,
40].
During both seasons, the total amount of effective rainfall was 35.2 and 16.1 cm respectively. Based on effective rainfall results, it clearly indicates that the irrigation water can be minimized during the off-season due to the high rainfall occurrence during that period. Lee et al. [
41] suggested constructing a storage facility to store the excessive flows caused by heavy rainfall events in order to augment rainfall whenever required. Rowshon et al. [
22] stated that use of rainfall is also essential in overcoming water shortages and improving the dependability of irrigation deliveries. Maina et al. [
23] conducted paddy rice water requirement experiment at sawah sempadan and reported total effective rainfall of 29.9 cm during the off-season, which is close to the value reported by the current study. Although they did not consider whole month of January, the heavy rain may occur during first two weeks of that month. However, almost same amount of effective rainfall was obtained during the month of February: 9.1 cm for the current study and 9.5 cm reported by [
23]. In Tanjung Karang Rice Irrigation Scheme (TAKRIS), the farmers practice conventional irrigation system, and field water level is maintained at a level of 3–10 cm until two weeks before harvesting and when water is about to drain. Thus, a continuous irrigation system was adopted, except during rainfall and when water level exceeded 10 cm. The highest irrigation supply occurred during mid-season when paddy plants grew and water crop water demand was high. During both seasons when there was heavy rainfall, there were no irrigation events. The total amount of irrigation water was 69.5 and 68.9 cm during off and main seasons, respectively. In the present study, drainage occurred only whenever the water level exceeded the outlet height of 10 cm and also during drainage periods. Thus, the total amount of drainage was higher during the off-season due to heavy rainfall events as compared to main season. During the off-season, drainage events occurred at end of January, February, and March when there were huge rainfall events (more than 3.5 cm) and also during field draining period. Our current finding suggests that drainage is largely dependent on both rainfall and irrigation. Thus, it is necessary to adapt water saving strategies in order to minimize excessive water losses from paddy fields [
42]. On the other hand, a large volume of water drained out from the study plot, especially during rainy days.
The mean rate of percolation water was 0.21 cm for the off-season and 0.18 cm for main season, respectively. It ranges from 0.17–0.28 cm day
−1 for the off-season and 0.12–0.25 cm day
−1 for main season, respectively. The deep percolation rate during the off-season was higher than that obtained during the main season. The mean rate of percolation water was 0.21 cm for the off-season and 0.18 cm for main season, respectively. It ranges from 0.17–0.28 cm day
−1 for the off-season and 0.15–0.25 cm day
−1 for main season, respectively. The deep percolation rate during the off-season was higher than that obtained during the main season. The percolation loss findings in the present study are almost close to those reported by other authors. Lee et al. [
41] found mean percolation rate of 0.27 cm day
−1 for the off-season and 0.22 cm day
−1 for main season, respectively. Ayob et al. [
43] reported daily average percolation value of 0.17 cm day
−1 at KADA Paddy Irrigation Scheme, Malaysia. However, it is much lower than those observed by other authors in Southeast Asian regions [
44,
45]. This is mainly due to the changes in the conditions of rice fields including soil texture and structure, top and subsoil thickness, standing water depth, water and soil temperature and salinity, depth to the groundwater table, and other topographical conditions [
40]. It is well understood that percolation rate increases as standing water depth in paddy plot increases.
Currently, there is a challenge to use less water with more production. To do so, water productivity index is the best indicator with which to express the value derived from the use of water. This concept comes from “more crops per drop”. Based on experimental results, the water productivity for irrigation (WPI) during the off-season and main season was 0.72 kg/m
3 and 0.78 kg/m
3 respectively. The water productivity for irrigation plus rainfall (WPIR) was 0.43 kg/m
3 for the off-season and 0.60 kg/m
3 for main season. However, the water productivity for evapotranspiration (WPE) was 1.03 kg/m
3 for the off-season and 0.98 kg/m
3 for main season, respectively. Usually, low water productivity indicates that the crop water requirement is high and opposite for high water productivity. Rashid et al. [
46] estimated water productivity during Boro and T.Aman seasons in Bangladesh by considering the total water input (irrigation and rainfall) and reported WP value of 0.58 and 0.49 kg/m
3, respectively. Li et al. [
32] conducted a field experiment under transplanted rice in China and investigated water productivity during two consecutive seasons. They reported WPI and WPIR values of 2.08 and 0.99 kg/m
3 during the first season and 3.85 and 0.77 kg/m
3 for the following season, respectively.
In Tanjung Karang paddy fields, farmers always use conventional irrigation system in which high water level was maintained during rice growing seasons, especially during reproductive stages. 8.5 cm to 10 cm standing water depth was recorded from mid-August to the end of September due to the frequent rainfall and irrigation supply events. Keeping water level above 7 cm may result in excess water loss from the paddy field. Therefore, in the present, we suggest that keeping the stagnant water depth around 4–6 cm will minimize the water losses both by deep percolation and surface drainage.