In addition to our estimates, we have used data from several of our institute's experimental stations and experimental reports on water requirements from irrigation experiment stations to analyze the water consumption of the principal cropping systems. Our results are listed in Table 2. From this table we can see that the long-run average annual water consumption of three crops in two years is the lowest, so this system is suited to the scarce and unevenly distributed precipitation in North China. In view of this, we must not overemphasize increasing the multiple cropping index in areas where irrigation conditions are inadequate but instead stress systems which are the most appropriate to the locality. This system can make full use of the region's moisture resources as well as its light and heat.
Winter wheat occupies a significant position in all the cropping systems discussed above.
It is therefore of great importance to study the patterns of winter wheat water consumption in order to understand field moisture conditions and even the field water balance in the North China Plain. The growing period for winter wheat is very long, about 240 days. Only a small amount of precipitation falls during this period and this can only satisfy onethird of the total water requirement, which is quite high, fluctuating between 400 and 550 mm. Of this, evapotranspiration constitutes about 60 to 70 per cent. It is about 180 days from sowing to greening up.