Proximal Sensing Methods for Mapping Soil Water Status in an Irrigated Maize Field
Approximately 80% of allocated freshwater in New Zealand is used for irrigation, and the area irrigated has increased by 55% every decade since 1965. The research described in this chapter therefore focuses on developing new techniques to map and monitor soil attributes relevant to irrigation water use efficiency. The apparent electrical conductivity (ECa) of soils under a 33-ha irrigated maize crop was mapped using a mobile electromagnetic induction (EM) and RTK-DGPS system, and this map was used to select three contrasting zones. Within each zone, further ECa values were recorded at a range of volumetric soil water contents (θ) to develop a relationship between ECa, soil texture, soil moisture, and available water-holding capacity (AWC) (R 2 = 0.8). This allowed spatial prediction of AWC, showing that these sandy and silty soils had similar AWCs (∼160 mm/m). High-resolution digital elevation data obtained in the EM survey were also co-kriged with TDR-derived θ to produce soil moisture prediction surfaces, indicating drying patterns and their relationship to topography and soil texture. There was a 12.5–13.1% difference in soil moisture to 45 cm soil depth between the wettest and the driest sites at any one time (n = 47). Spatial and temporal variability of soil moisture, indicated by these co-kriged prediction surfaces, highlights the need for a rapid high-resolution method to assess in situ soil moisture. The potential of soil spectral reflectance (350–2,500 nm range; 1.4–2 nm resolution) for rapid field estimation of soil moisture was therefore investigated. Soil spectra were pre-processed and regressed against known soil moisture values using partial least squares regression (R 2 calibration = 0.79; R 2 prediction using leave-one-out cross-validation = 0.71). These proximal sensing methods facilitate spatial prediction of soil moisture, information which could then be uploaded to a variable rate irrigator.
KeywordsAvailable water-holding capacity Co-krige Digital elevation map EM mapping
The authors would like to thank Hew and Roger Dalrymple for use of their farm. The research has been funded by the Agricultural and Marketing Research and Development Trust, New Zealand (AGMARDT), the New Zealand Vice-Chancellors’ Committee, William Georgetti Trust, and The Sustainable Land Use and Research Initiative (SLURI), New Zealand.
- Hedley CB, Yule IJ, Bradbury S (2005) Using electromagnetic mapping to optimise irrigation water use by pastoral soils. Proceedings of the joint international conference of the New Zealand Hydrological Society, International Association of Hydrogeologists, and New Zealand Society of Soil Science, Auckland, 28 November–2 December, New Zealand Hydrological Society, Wellington, New Zealand, CD-ROMGoogle Scholar
- Kaleita AL, Tian LF, Hirschi MC (2005) Relationship between soil moisture content and soil surface reflectance. Trans Am Soc Agric Eng 48(5):1979–1986Google Scholar
- Milne JDG, Clayden B, Singleton PL, Wilson AD (1995) Soil description handbook. Manaaki Whenua Press, Landcare Research, New Zealand, 157 ppGoogle Scholar
- Waine TW, Blackmore BS, Godwin RJ (2000) Mapping available water content and estimating soil textural class using electromagnetic induction. EurAgEng 2000, Warwick, UK, Paper 00-SW-044, European Society of Agricultural Engineers, UKGoogle Scholar