EFFECTS OF WATER AVAILABILITY AND VINE HARVESTING FREQUENCY ON THE PRODUCTIVITY OF SWEET POTATO IN SOUTHERN MOZAMBIQUE. II. CROP WATER USE

Abstract

In Mozambique, the sweet potato (Ipomoea batatas) is grown in the dry season in areas with a high water table, or as an irrigated crop. It is also grown in the rainy season when short-term droughts can occur. This paper reports the results of experiments designed to evaluate the effects of water availability and vine-harvesting practices on actual crop water use (cv. TIS 2534), during two contrasting seasons in the south of the country. Rain-fed crops extracted water to soil depths (variable sandy loam) of 0.75 m, with most (90%) taken from the top 0.25 m. Well-irrigated crops apparently obtained some (10%) water from below 0.75 m, but most (75%) came from the upper 0.25 m. Total water use from well-watered crops was about 800 mm during the rains, and 550 mm during the dry season. The corresponding values for rain-fed crops were 360 and 180 mm respectively. Peak rates of water use averaged 8 mm d⁻¹ in the rainy season, and 5–6 mm d⁻¹ in the dry season. Water use was not modified by the frequency of vine harvests. Actual rates of evapotranspiration fell below maximum values when the soil water deficit exceeded only 20 mm. By comparison, the maximum depth of extractable water in the root zone was 80–90 mm. The crop coefficients (Kc) for each stage of growth were consistent over both seasons and estimated to be: 0.55–0.7 (crop establishment), 1.1–1.2 (mid-season), and 0.8 (end-season).