Evaluation of different methods for gap filling of long‐term actual evapotranspiration time series measured by lysimeters

Abstract

Terrestrial evapotranspiration (ET) is the second largest water flux in the global water cycle. It can be measured with different techniques; weighable lysimeters can provide very accurate measurements, and some very long-term time series exist. However, these lysimeter time series are affected by data gaps that must be filled to estimate actual ET totals and long-term trends. In this paper, we explore four different gapfilling methods: the potential ET-method, the ratio method, the FAO-based water balance method, and HYDRUS modeling. These gap-filling methods were evaluated for three time series of actual ET measured by lysimeters and meteorological data of three European sites. Separate evaluations were made for the five driest and five wettest April-October periods to investigate whether the performance of the gapfilling methods was affected by hydrological conditions. Series of random gaps were artificially created for the three time series, including gaps of four different lengths. Actual ET was estimated for these gaps with the gap-filling methods, which were evaluated based on RMSE and mean bias error. The results show that the ratio method outperformed other methods for gap filling of lysimeter data for Basel (Switzerland), whereas the HYDRUS method outperformed other methods for Rheindahlen (Germany). For Rietholzbach (Switzerland), the different methods performed very similarly, except that the FAO method gives slightly larger RMSEs. The gap-filling methods do not perform very differently for dry and wet conditions. The ratio method is recommended for filling smaller gaps, and the HYDRUS method is recommended for longer gaps of 30 d.

Long-term trends in agricultural droughts over Netherlands and Germany: how extreme was the year 2018?

“…Various models have been used to assess droughts (Mishra and Singh, 2011). In this study, the HYDRUS-1D version 4.17 model (Simunek et al, 2005;Šimůnek et al, 2008;Huang et al, 2020) was applied to calculate actual ET, soil moisture and resulting drought statistics. We simulated uniform water flow in 2 meters deep soil columns with homogeneous soil texture and a root uptake sink term.…”

A low-cost simple lysimeter soil retriever design for retrieving soil from small lysimeters

“…The lysimeter is an instrumented vessel containing a soil column and is the basis of the standard method for estimating crop evapotranspiration (Howell et al 1995, Huang et al 2020. It is also used in a wide range of agricultural and environmental studies including the movement of chemicals in the vadose zone (Pütz et al 2018, Belik et al 2021, measuring hydrological components (Brown et al 2021), movement of microbes in soil (Jiang et al 2010), plant-soil interactions (Portela et al 2024) and investigations into environmental pollution (Goss et al 2010).…”

Sensitivity of runoff due to changes in the characteristics of the water balance in the Danube River region

“…Estimates of evapotranspiration are usually based on on-site experimental assessments (e.g., evaporimeters and lysimeters) (Hobbins et al, 2004;Huang et al, 2020), spatial estimates using, e.g., remote sensing data (Gao et al, 2010), or outputs from hydrological rainfall-runoff models (e.g., Parajka et al, 2004;Zhang et al, 2010). In addition, spatial interpolation methods varying complexity from point estimates of actual evapotranspiration can also be used (Hobbins et al, 2001;Lapin et al, 2016).…”