Part I, largely introductory, outlines the design of an experiment to measure the responses of ordinary farm crops to supplementary irrigation in a part of England where, from meteorological reasoning, the frequency of need was expected to be about 2 years in 3. The watering treatments were based on estimates of potential transpiration, made from weekly weather data collected on the site, and measured rainfall. The basis of the computation is restated, and the balance of rain and estimated transpiration plotted for each of the 9 years, week by week in summer, month by month in winter. The meteorological specification of frequency of need was satisfied in 5 of the years, during which twenty crops were taken. Of these, eighteen responded to irrigation. In the other years, four of the sixteen crops responded positively to irrigation, and a few of the remaining twelve responded negatively when early irrigation was followed by summer rainfall sufficiently in excess to cause leaching from irrigated plots.