Retrieving Soil Moisture Storage in the Unsaturated Zone Using Satellite Imagery and Bi-Annual Phreatic Surface Fluctuations |
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| Authors: | Mobin-ud-Din Ahmad WGM Bastiaanssen |
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| Institution: | (1) International Institute for Geo-information Science and Earth Observation (ITC), P.O. Box 6, 7500 AA Enschede, The Netherlands;(2) Regional Office for Pakistan, Central Asia and Middle East, International Water Management Institute (IWMI), 12 km Multan Road, Chowk Thokar Niaz Baig, Lahore, 54700, Pakistan;(3) WaterWatch, Generaal Foulkesweg 28, 6703 BS Wageningen, The Netherlands |
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| Abstract: | Ongoing research into net groundwateruse in Pakistan needed soil moisturestorage information to compute monthlysoil water balances. A methodology isdeveloped to retrieve soil moisture storagein the complete unsaturated zone from rootzone soil moisture content (based onthermal satellite imagery) and bi-annualphreatic surface fluctuations. A root meansquare error in volumetric soil moisturecontent of 0.05 cm3 cm-3 in theroot zone of irrigated fields was found forthe case study in Pakistan. A new simpleparameterisation of matric pressure headdistribution between the root zone and thephreatic surface was developed. Theabsolute root mean square error in dailyestimates of unsaturated zone storage forshallow (2 m) and deep groundwater tables(10 m) was found to be 7 cm (the averagetotal storage is 110 cm). It is concludedthat the spatial variation of the soilmoisture storage depends on the depth ofthe phreatic surface, whereas the temporalvariation is mainly controlled by the rootzone soil moisture changes. The resultsshow that for an area of 3 million ha, storagechanges of ± 10 cm month-1 occur,which is a significant quantity for monthlywater balance analysis. Conventionalmethods such as specific yield do notconsider moisture changes in the irrigatedtop soil when the groundwater table isdeep. The new method is, therefore, apossible alternative solution, especiallyin areas where hydrological data isscanty. |
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| Keywords: | evaporative fraction Pakistan phreatic surface Rechna Doab remote sensing soil moisture content soil moisture storage unsaturated zone |
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