Abstract
Monitoring volumetric soil water content (θv) is the key for assessing water availability and nutrient fluxes. This study evaluated the empirical accuracy of θv measurements using standard and in situ calibrated frequency domain reflectometers (FDR) with gravimetric water content and bulk density measurements of 1512 samples gathered from 15 profiles across 5 ICP Forests level II intensive monitoring plots. The predicted θv, calibrated with standard functions, predominantly underestimated the real water content. The measurement error exceeded the threshold of 0.03 m3 m−3 in 93% of all soil layers. Layer specific calibration removed bias and reduced the overall prediction error with a factor up to 2.8. A simple linear regression often provided the best calibration model; temperature correction was helpful in specific cases. To adequately remove bias in our study plots, a calibration dataset of up to 24 monthly observations was required for topsoils (whereas 12 observations sufficed for subsoils). Based on estimated precision errors, 3 sensors per soil layer proved to be sufficient, while up to 16 sensors are needed to meet the required accuracy in organic topsoils. Validating FDR sensor outputs using in situ gravimetric measurements is essential for quality control and assurance of long term θv monitoring and for improving site specific instrumentalization.
Highlights
The Campbell CS616 sensor, we evaluated in this study, is an frequency domain reflectometers (FDR) type sensor widely recognized as a robust probe suitable for long-term monitoring, less accurate than time domain reflectometry (TDR) but more cost-effective [13]
Cooperative Program (ICP) Forests (Working Group on Effects, UNECE AIR convention) is recommending in its current manual [19] a minimum acceptable accuracy for θ v measurements of 0.03 m3 m−3. We evaluated whether this accuracy level is achievable at single FDR sensor level using the standard calibration functions proposed by the manufacturer [20] or by recalibration functions using the direct field approach
From the three profiles studied per plot (Figures A1–A5 and Figures S1–S5), we found indications for inhomogeneous soil properties affecting soil moisture measurements and the principal components analysis confirmed that ρb, total organic carbon (TOC) and clay content are important controls
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Volumetric soil water content (θ v ) is a key variable in hydrological, climatological, and environmental studies and its monitoring is essential to investigate soil-water-plant relationships and nutrient fluxes. The need for accurate spatiotemporal soil moisture data in all types of soils and land-use is even increasing. Inaccurate soil moisture data has several potential implications: it may lead to the development of unreliable indicators for soil water availability or drought, it will introduce noise and systematic errors in water balance models at various spatial scales; and it could lead to incorrect nutrient budget calculations. There is a great need for affordable, highly accurate soil water measurement systems operating at a low cost for a long time
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