In situ measured soil moisture in China Soil moisture measurement in China

In China, soil moisture measurement started from 1980s (mostly from 1981), which aimed to serve the purpose of research and application of agricultural meteorology. Soil water condition (in-situ soil moisture) is the main observation element.

There are two techniques for soil moisture measurement at agricultural meteorology stations: the gravimetric method, and neutron probes. The gravimetric method is also called the thermostat-weight technique. Soil samples are taken using coring devices or augers at required depths and locations. The sample is weighed, oven-dried, and weighed again. The difference in mass gives the total soil moisture in the sample, which is converted to volumetric units with the density of the soil. The neutron probes method is to install a probe with a fast neutron source on the surface or lower soil in an access tube (transparent to the neutrons), and the backscat-tered flux of slow neutrons are measured, which is proportional to the density of hydrogen atoms. Because water is the major source of hydrogen atoms that changes with time, the neutron probe can provide a good measurement of soil water content. Calibration of slow neutron counts with gravimetric samples of soil moisture content and bulk densities yields a relationship to estimate the volumetric soil moisture content.

The in-situ soil moisture is measured at 11 layers of 0-5cm, 5-10cm, 10-20cm, 20-30cm, 30-40cm, 40-50cm, 50-60cm, 60-70cm, 70-80cm, 80-90cm and 90-100cm and on 8th, 18th and 28th days every month, except for winter time in northern China where soil is frozen. Detailed information can be found in the observational method and criterion (National meteorological administration, 1993). There are 178 gauge stations available at present with 76 new gauge stations established in the west, and distributions are shown in Fig. 7.2.

Fig. 7.2 Distributions of gauge stations for soil moisture measurement in China

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