In monsoon climates, the mean daytime saturation vapour pressure deficit (D), measured at screen height, decreases from a maximum of 3–4 kPa attained several months before rain arrives to a minimum of 0.5–1 kPa in the month of highest rainfall. Climatic records from India and West Africa were analysed to give the relation between D and precipitation (P mm month−1) as:
where n ranged from 0.5 to 0.9 between stations.
The dependence of dry matter production on D and therefore on P is evaluated when growth is restricted by rain and when this restriction is removed by irrigation. In the first case, the decrease in demand for water associated with a decrease in D is comparable in importance with the increase of water supply (P). The analysis has implications for the marginal response of crops to supplemental irrigation and for the interpretation of experiments with a line-source or rain-shelter.