This International Standard specifies an in situ method for the determination of water content in the unsaturated zone of soils using a neutron depth probe. lt is applicable when investigations into the water storage, water balance and water distribution in the unsaturated zone of the soil are carried out. Because the method is non-destructive, it is particularly suitable for repeated measurements at fixed locations. Water content profiles tan be obtained by measuring at a series of depths down to any depth within the range of the phreatic level at the site. The advantage of the method compared with some others, for example the gamma probe method, is the rapidity with which measurements tan be carried out. A disadvantage, however, is the relatively poor depth resolution of the measurements.