Mature lucerne on shallow granite sand was irrigated at different intervals, determined by four factors, 0·4, 0·6, 0·8 and 1·0, applied to daily evaporation (E0) as measured by a USWB Class A pan. Moisture balances were calculated for each treatment, and when the accumulated sum of daily evaporation, multiplied by the appropriate factor, reached 1·2 inches, which was about 50 per cent of the available moisture in the top 2 feet of soil, this amount of water was applied to the plots of the treatment concerned. Irrigation on the basis of 0·8 E0 gave the but overall results, although the highest yield from a single cut was obtained with the irrigation frequency defined by 1·0 E0. During the rainy season yields from all treatments declined markedly, those from the most frequently irrigated treatment showing the greatest drop. The lucerne under the irrigation regimes defined by 0·4 and 0·6 E0, although subjected to considerable moisture stress in the dry weather, suffered no lasting ill effects and yielded well during the rains. The experimental area was almost certainly subject to advective energy but the results are considered valid for local conditions.