Figure 4. Average NDVI integral (NDVI-I) (a) and date of maximum NDVI (DMAX) (b)  for the period 1990-1996 for the four land use categories: irrigated crops (I), non-irrigated crops (NI), grasslands (RL) and Conservation Reserve Program (CRP). Different letters indicate significant differences (ANCOVA, p<0.001, see Table 2).