**1. Introduction**

Rice is the main staple food in Indonesia and many countries worldwide; its demand has steadily increased in recent decades as the population grows. Water availability is a key component for rice production, but its sustained availability remains uncertain, due to increased water use from other sectors [1], thus threatening the irrigation water supply [2,3]. Moreover, current practices of continuously flooded farming have worsened water availability. Although conventional flooded irrigation systems may increase yield [4], their design is not efficient, which reduces water productivity [5] and promotes more greenhouse gas emissions, especially methane gas [6].

Intermittent irrigation is an alternative irrigation strategy that typically saves more water and is sometimes integrated with an adaptive rice farming called System of Rice Intensification (SRI) [7]. Previous studies have proven that SRI application increased rice yield [8,9], thus raising water productivity [10–12]. SRI is also recognized to be more

**Citation:** Arif, C.; Saptomo, S.K.; Setiawan, B.I.; Taufik, M.; Suwarno, W.B.; Mizoguchi, M. A Model of Evapotranspirative Irrigation to Manage the Various Water Levels in the System of Rice Intensification (SRI) and Its Effect on Crop and Water Productivities. *Water* **2022**, *14*, 170. https://doi.org/10.3390/ w14020170

Academic Editor: Alban Kuriqi

Received: 20 November 2021 Accepted: 4 January 2022 Published: 8 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

environmentally friendly [13] because of its ability to suppress methane gas emission [14,15], which potentially reduces global warming potential (GWP) [16]. Therefore, this system is an appropriate choice for climate change adaptation and mitigation strategies [17] for the agricultural sector. By using the system, the field does not need to be flooded continuously, but rather it is possible to lower the water table and water level below the soil surface [18,19].

The main challenge in implementing the SRI system, like other precision farming techniques, is how to precisely control the water levels at the field, especially for farm-level farmers. So far, precision farming usually identically relates to irrigation automation that requires more cost investment of automated instruments and wireless sensor networks [20]. The idea requires installing water content sensors on rice fields, sending data wirelessly from the sensors to the running controller, then carrying out actions in opening/closing solenoid valves at the irrigation inlet [21]. Obviously, the technology is too expensive and is very difficult to be implemented by farmers. The technology may only be applied to farmsteads or agricultural industries with more capital to invest in technology and human resources.

In principle, the implementation of precision and smart agriculture does not necessarily require advanced automatic control technology. The principle is "to provide the right input, at the right place, at the right time, in the right amount, in the right way, using the right tools" [22]. Therefore, the implementation of precision farming remains a research challenge, especially when dealing with an applicable-efficient irrigation technology. Here, we propose a model, evapotranspirative irrigation technology. Principally, the field is watered based on the actual evapotranspiration rate by operating a simple automatic floating valve. The valve will automatically open or close with mechanical principles according to the desired water level; this idea is more straightforward than piped irrigation systems [23]. The inlet holes in the piped irrigation systems are replaced with float valves, while irrigation canals are replaced with pipelines and high investment costs. While the concept of evapotranspirative irrigation does not require modification of the irrigation canals, it modifies the inlet valve with an automatic float system. This system does not need electric power, but uses a simple mechanical principle to open and close the valve.

Theoretically, an evapotranspirative irrigation system is elaborated from the concept of evaporative irrigation [24]. The functional design has been designed and developed [25], as well as tested for lettuce plants [26]. However, the system has not been tested with various irrigation regimes with different water levels under specific weather conditions for SRI paddy cultivation. Subsequently, its effect on crop and water productivities would require observation. Therefore, the objectives of this study were to (1) evaluate the performances of evapotranspirative irrigation with different irrigation regimes under specific weather condition, and to (2) observe crop and water productivities on SRI paddy cultivation at different regimes.

#### **2. Materials and Methods**

#### *2.1. Study Site*

The research was a laboratory-scale experiment, which was conducted in one rice planting season from July to November 2020. We carried out an experiment in the Kinjiro Farm (coordinates 6.59◦ S, 106.77◦ E), Bogor, West Java, Indonesia. Rice seed was sown on 5 July 2020 and was planted on 19 July 2020. After 112 days of cultivation, the grains were harvested on 10 November 2020.

In a preliminary study, we sampled soils with three replicates on 0–30 cm. From the samples, we obtained information on soil properties at the study site. Typically, soil texture was characterized as a clay loam with a silt content more than 40%. Detailed soil physical properties are presented in Table 1.


**Table 1.** Soil physical properties of the field location.

Note: Three soil samples were collected and were analyzed in a certified laboratory. The data in the table are the mean ± SD.

#### *2.2. Experimental Design*

The model of evapotranspirative irrigation was applied in the lab-scale experiment. A miniature paddy field with the dimensions of 4 m × 4 m and 0.5 m in height was used for each irrigation regime (Figure 1). Additionally, there was drainage storage with the dimensions of 0.5 m × 2 m × 0.5 m connected to the outlet's miniature paddy field model. However, the drainage was not controlled, and the water flowed naturally. In the inlet, there was a simple automatic float valve. The valve is equipped with a floating cylinder that pushes the valve upward when the water level rises to a particular level, thereby closing the valve. On the other hand, when the water level drops (in this case caused by evapotranspiration), the floating cylinder will also go down, caused by the valve opening (Figure 1). The bucket was covered with a transparent fiberglass cover to minimize evaporation.

**Figure 1.** A miniature paddy field equipped with a simple automatic valve.

Here, we applied three water irrigation regimes with two replications, so there were six miniature paddy fields. The first regime was continuous flooded irrigation (CFI), where flooded water with 0–4 cm water level above the soil surface (the setpoint at 2 cm) was applied during planting season as control. The second regime was moderate flooded irrigation (MFI), where applied shallow flooded water with 0–2 cm water depth was used (the set point at 0 cm). The last regime was water-saving irrigation (WSI), which kept the water level at the soil surface (the set point at 0 cm) for 0–20 days after transplanting (DAT), and then dropped 5 cm below the soil surface (the setpoint −5 cm). The WSI was selected based on the previous finding that revealed that the optimum water level for SRI to mitigate greenhouse gas emission was 5 cm below the soil surface [19].

In the rice cultivation, there were some components adopted, such as planting young seedlings (14 days after sowing), adding space between hills of 30 × 30 cm<sup>2</sup> and placing a single plant in each hill. This practice is known as the System of Rice Intensification (SRI), as commonly applied in Indonesia [27]. For the fertilizer application, all plots were supplied with the same doses, i.e., a combination of organic (1 ton compost/ha) and inorganic (100 kg/ha of urea, 75 kg/ha of phosphorus and 50 kg/ha of KCl) fertilizers.
