**1. Introduction**

Water is a valuable resource that is becoming increasingly scarce due to growing population and intensifying agriculture [1]. Water scarcity is challenging the ability of countries to meet the increasing food demand [2]. Globally, agriculture is the largest consumer (≈70%) of freshwater accounting for 90% of consumptive water use [3,4]. Of the three main food crops (maize, wheat, and rice), rice is the most important crop especially in developing countries [5]. Given its large water footprint, practices that can reduce water inputs for rice production such as deficit irrigation need to be explored. Deficit irrigation

is a technique used to minimize water losses and increase water efficiency, especially in areas where there is insufficient water supply for irrigation. Deficit irrigation management involves inducing marginal stress, except in critical growth stages where crop yield might be negatively affected [6].

Expansion of rice growing area is larger than any other crop in Africa due to its increasing demand. [7]. Tanzania is the largest (947,303 km2) country in East Africa and accounts for 9% (2.6 million ton) of African rice production (30.8 million ton) [8]. However, due to a rising gap between production and consumption, many African countries, including Tanzania, are becoming increasingly dependent on rice imports [9,10]. At the same time, increasing irrigation withdrawals and spatial and temporal variability in rainfall and surface flows are causing water scarcity in many parts of Tanzania such as the Pangani and Rufiji River basin [11,12]. The Pangani and Rufiji rivers support majority of irrigated agriculture in Tanzania and support almost entire hydroelectric generation in Tanzania (Mtera, Kidatu and Kihansi plants) [12]. Growing population, increasing food demands, and increased rainfall variability due to changed climate is likely to exacerbate water availability in the future. There is a need to develop alternative farming systems that can increase or sustain rice yields with reduced water footprint to ensure the food security in Tanzania.

Field water use for rice typically ranges from 1000–2000 mm [13], which is 2–3 times of other cereal crops. In rice production systems, a large quantity of water is lost through evapotranspiration, surface runoff, seepage, and deep percolation [14]. Several water-saving irrigation techniques have been developed for rice [15]. For instance, in Asia, the most widely adopted water-saving practice is aerobic rice production system. Although the aerobic rice system reduces water use it also results in lower yields compared to lowland flooded rice [16]. This practice also has some limitations related to soil type, rice variety, and socio-economic constraints [17]. Other strategies being pursued to reduce rice water requirements include alternate wetting and drying (AWD) and saturated soil culture [18]. Studies show that AWD can reduce crop water requirements while maintaining or even increasing the yield as compared to the conventional flooded system [19,20]. The AWD is an irrigation practice where water is applied to attain certain depth of ponding after which the field is left unirrigated for some time (e.g., 5 to 7 days) to dry out or drain. In the traditional continuously flooded (CF) system, water is applied at a frequency that will maintain a certain depth (e.g., 5 cm) of ponding throughout the season. Under the CF, more than 50% of irrigation water is lost through seepage, deep percolation, and excessive unproductive evaporation [21].

The system of rice intensification (SRI) is a relatively new production practice that has been adopted by many farmers in Asia and Sub-Saharan Africa [22–25]. The SRI is a combination of agronomic practices comprising of land preparation, seed selection, nursery establishment, transplanting of young age seedlings (8 to 12 days), wider plant spacing, AWD, and frequent weeding [22,26]. The SRI practice has been reported to substantially increase the yields as compared to the CF system [27]. The AWD irrigation technique used in the SRI production system can reduce water use by minimizing evaporation and deep percolation losses. The SRI combined with AWD has a potential to reduce water application and yet increase or sustain current yields in Tanzania. A range of AWD regimes are possible with SRI. However, limited research has been conducted on the evaluation of SRI in combination with different AWD regimes (e.g., ponding depth) in Tanzania. The current SRI practice lacks specific information regarding the irrigation management needed to achieve optimum yield. The goal of this study was to find water sustainable rice production systems in Tanzania by comparing water and yield metrics of conventional continuously flooded rice with SRI production system under different AWD regimes during wet (February-June) and dry (September-January) seasons. Specifically, the study attempts to answer the following questions: (a) Can SRI-AWD combination with reduced ponding depth significantly increase crop yield and reduce irrigation requirement compared to CF? (b) Which irrigation management strategy (scheduling and ponding depth) provides highest water productivity?
