*2.1. Experimental Sites*

The field experiments were conducted with lettuce plants (*Lactuca sativa* var. *crispa L.*) which are widely used in the study area, during a period from August to September 2017 in two experimental sites located in the villages of Chea Rov (site S1) (104◦38 54.442 E 12◦9 15.482 N) and Ou Roung (site S2) (104◦37 16.24 E 12◦11 52.518 N) in the province of Kampong Chhnang, Cambodia (Figure 1).

**Figure 1.** Experimental sites at Chearov (S1) and Ou Roung (S2), located in the Chrey Bak Catchment.

The total land area of the plots was 400 m2. Lettuce seeds were sown in standard trays (with 123 holes). After 15 days, seedlings were transplanted into raised bed rows (0.30 m in height and with bed tops 0.50 cm wide) and covered with plastic mulch with a planting density of 12 plants m<sup>−</sup>2. The compost was basally applied at the rate of 20 ton ha−<sup>1</sup> before transplantation.

Irrigation was carried out using a drip system, with emitters of constructor maximum discharge of 3 L h−<sup>1</sup> spaced 0.10 m apart. A plastic cover was used to protect the crops from heavy rainfall. Nevertheless, due to the intense rain which flowed between the crop rows, water ponding at 20 cm below the top bed row level was observed between the lettuce rows at both sites during almost the entire growing period. This ponding kept the soil wet during the growing period, and had to be factored into the calibration of the lettuce growing curve. At site S2, irrigation was not applied after a week after planting, due to the benefit of water ponding. At site S1, even though there was also water ponding in the field, the irrigation was applied every other day. The irrigation was determined by checking soil moisture (SM) using the feel and appearance method of Klocke et al. [68]. The irrigation was done when the SM was depleted below field capacity in the root zone at 5 cm, as lettuce have a root depth between 5–10 cm.
