Water stress is present when actual transpiration from agricultural crops falls short of the potential transpiration, derived from atmospheric conditions and from crop leaf expansion. We present here an agricultural drought index based on the daily transpiration deficit and its integration over time. A water balance model developed by us in the past is used for computing daily potential and actual transpiration for a representative crop (e.g. alfalfa). Daily values of the transpiration deficit (i.e. Potential - Actual Transpiration) for the current year are integrated over time for 30, 60, ..., 240 days before present and the values so obtained are compared with ranked transpiration deficits for the same day of year in the local climatic time series (usually 50 years, 1951-2000). The drought index coincides with the percentile P of the current value computed as P=R/(N+1) where R is the rank of the current year and N is the number of years available in the time series. The values 100 and 0 correspond to current values respectively higher or lower than any value present in the reference time series. Examples are presented for year 2003 in Emilia-Romagna, an agricultural region of northern Italy.