Research On Distributed Water Content Measurement In Soil Profile Based On Time Domain Reflectometry

Soil water content is a very important indicator in the fields of agricultural science,hydrological science and meteorological science,so the accurate determination of soil water content is of great importance.The current application of time domain reflectometry(TDR)to measure soil water content is to use a certain thickness of soil as a measurement layer and use TDR to measure the average water content of the soil layer within the delineated area.This totalized measurement method,in which the average water content measured by the sensor represents the water content of the entire soil layer,does not reflect the moisture variation at specific depths of the soil.For this reason,this paper proposes a distributed water content measurement method in which the sensor is inserted vertically into the soil.By designing and conducting soil column experiments and theoretical studies,we investigate the connection between the soil impedance waveform and the water content in the vertical direction of the soil,and verify the feasibility and accuracy of the method through drip irrigation experiments.The main research work and conclusions of this paper are as follows.(1)Using the impedance controllability and low cost of PCB,two types of PCB-type soil sensor probes with salt resistance were designed:DT-1 probe and FT-1 probe.After comparing them with the conventional steel pin probes through water-air interface contact tests at different depths and probe salt resistance tests,it was found that the DT-1 probe performed significantly better than the other two probes,so it was used as the probe for the soil distributed water content measurement experiments in this study.(2)Dielectric measurements of layered media by physical properties and dielectric constant relationships of two liquids,edible oil and pure water.It is shown that TDR100 time domain reflectometer and VNA-based V-TDR and S-TDR methods can measure layered media,and S-TDR is superior in the continuous measurement of layered media.The S-TDR was used to make experimental measurements of infiltration line depths of 0cm,4cm,8cm,12cm,16cm and 20cm for four different textured soils,namely Shaanxi small mound soil,yellow sheep soil,clay loam soil and black soil,and a soil infiltration line measurement model was established by fitting the measured depths to the actual set depths.The model was validated using three layers of soil experiments with R~2=0.9869,which was a good fit.(3)Linear,second-order and third-order fits of the distributed soil water content were carried out using T_h and L_hwhich measured by TDR100 time domain reflectometer,V-TDR and S-TDR.The results show that the R~2 of the third-order soil distributed water content measurement models constructed by the three methods are 0.9753,0.9562 and 0.9514respectively,and the p-value of the soil distributed water content measurement by the soil distributed water content measurement models constructed based on V-TDR and S-TDR is less than 0.0001.By setting up a three-layer soil moisture content measurement experiment,the S-TDR-based soil distributed moisture content measurement model yielded the highest correlation with the actual soil moisture content measured by the drying method,making it the optimal method for soil distributed moisture content measurement.Finally,the soil column infiltration experiment was carried out by drip irrigation on Shaanxi Lou soil and black soil.The moisture content of the drip-irrigated soil was compared with the soil moisture content obtained from the S-TDR-based distributed soil moisture content measurement model after drying in 10 layers.The results show that the empirical formula for measuring distributed soil moisture content based on S-TDR has a high accuracy of less than 2.5%from the actual value when measuring the moisture content of the soil in each layer after drip irrigation.Also,in order to discuss the minimum measurement range of this measurement model,this paper calculates and analyses each sampling point of the soil impedance waveform after drip irrigation and finds that the error of this model for each layer of soil water content is less than 2.23%when the soil depth is greater than 7.2cm.Therefore,the measurement model can be used for continuous monitoring of soil moisture in smart irrigation.Moreover,with the development and promotion of portable VNA,the VNA-based S-TDR soil distributed water content measurement method can provide a new way of thinking for soil moisture information acquisition.