In the past few years, the interest in growing crops and trees for bioenergy production has increased. One agricultural practice is the mixed cultivation of fast-growing trees and annual crops or perennial grasslands on the same piece of land, which is referred to as one type of agroforestry (AF). The inclusion of tree strips into the agricultural landscape has been shown – on the one hand – to lead to reduced wind speeds and higher carbon sequestration above ground and in the soil. On the other hand, concerns have been raised about increased water losses to the atmosphere via evapotranspiration (ET). Therefore, we hypothesise that short rotation coppice agroforestry systems have higher water losses to the atmosphere via ET compared to monoculture (MC) agriculture without trees. In order to test the hypothesis, the main objective was to measure the actual evapotranspiration of five AF systems in Germany and compare those to five monoculture systems in the close vicinity of the AF systems.

We measured actual ET at five AF sites in direct comparison to five monoculture sites in northern Germany in 2016 and 2017. We used an eddy covariance energy balance (ECEB) set-up and a low-cost eddy covariance (EC-LC) set-up to measure actual ET over each AF and each MC system. We conducted direct eddy covariance (EC) measurement campaigns with approximately 4 weeks' duration for method validation.

Results from the short-term measurement campaigns showed a high agreement between

With respect to the annual sums of ET over AF and MC, we observed small differences between the two land uses. We interpret this as being an effect of compensating the small-scale differences in ET next to and in between the tree strips for ET measurements on the system scale. Most likely, the differences in ET rates next to and in between the tree strips are of the same order of magnitude, but of the opposite sign, and compensate each other throughout the year. Differences between annual sums of ET from the two methods were of the same order of magnitude as differences between the two land uses. Compared to the effect of land use and different methods on ET, we found larger mean evapotranspiration indices (

We conclude that agroforestry has not resulted in an increased water loss to the atmosphere, indicating that agroforestry in Germany can be a land-use alternative to monoculture agriculture without trees.

In the past few years, the interest in growing crops and trees for the production of bioenergy has increased, especially in the scope of climate change mitigation and carbon sequestration

The cultivation of fast-growing trees with annual crops or perennial grasslands on the same piece of land is an example of agroforestry (AF)

Evapotranspiration (ET) in AF is strongly affected by the tree strip properties and is the combined process of (1) evaporation from the soil and open water from leaf surfaces and (2) leaf transpiration

Currently, little is known about the system-scale water use of heterogeneously shaped short rotation alley cropping agroforestry systems in Germany. The majority of the previous studies focused on the water use of short rotation coppices, but not on AF systems

However, the effect of AF on system-scale evapotranspiration is site specific and depends on the local climate, soil type, water availability, and AF design. Therefore, repeated measurements at different sites are essential for studies on the effects of AF on evapotranspiration. Nevertheless, this requires low maintenance methods with low power consumption and a moderate cost.

The most common approach for evapotranspiration measurements at ecosystem scale is the eddy covariance (EC) method

The main hypothesis of the current work is that short rotation alley cropping AF systems have higher water losses to the atmosphere via ET, compared to monoculture agriculture without trees. In order to test the hypothesis, the main objectives of the study are (1) to evaluate the eddy covariance energy balance (ECEB) and low-cost eddy covariance (EC-LC) method against direct eddy covariance (EC) measurements and (2) to measure the actual evapotranspiration of five AF systems in Germany and compare those to five monoculture systems in the close vicinity of the AF systems using the two different approaches.

This study was carried out as part of the sustainable intensification of agriculture through agroforestry (SIGNAL) project (

Map of the SIGNAL sites, with the respective agroforestry (AF) system type of either cropland or grassland AF, and an image and aerial photograph of the AF systems. Green hatched areas in the aerial photographs correspond to the area of the AF system, and red hatched areas correspond to the area of the MC system. Site images are our own photographs, and the aerial photographs originate from Google Maps and Google Earth. © Google 2020.

Site locations and agroforestry (AF) system geometry.

Measurements of meteorological and micrometeorological variables have been performed since March 2016. At each AF system we installed an eddy covariance mast with a height of 10 m, and at each MC system an eddy covariance mast with a height of 3.5 m was installed. Each mast was equipped with the same meteorological and micrometeorological instrumentation. The standard set-up consisted of instruments measuring wind speed, wind direction, sensible heat flux, net radiation, global radiation, air temperature, relative humidity, precipitation, and ground heat flux. An overview of the installed instruments and the respective variables used for the presented set-ups is given in Table

Gaps in precipitation measurements at all sites were filled by precipitation data collected at nearby weather stations operated by the German weather service (DWD). We used the R package of rdwd

In the following sections, we briefly describe the concepts of the used set-ups, eddy covariance (EC), eddy covariance energy balance (ECEB) and low-cost eddy covariance (EC-LC). Throughout the paper we used the respective abbreviations.

Instrumentation for flux and meteorological measurements used at all five AF and MC systems. Set-up corresponds to eddy covariance (EC), low-cost eddy covariance (EC-LC), and eddy covariance energy balance (ECEB).

Sensible heat and momentum fluxes have been measured continuously with ultrasonic anemometers since 2016. The water vapour and CO

The energy balance at the surface is as follows:

LE from ECEB (

The energy balance residual (

The EC-LC set-ups comprised the same ultrasonic anemometer uSONIC-3 Omni as used for the EC and ECEB set-ups plus a compact, low-cost relative humidity, air temperature, and pressure sensor (BME280; Bosch, Germany; see Table

For the comparison of

For the calculation of annual sums of

The calculated residual was treated as the dependent variable, whereas the net radiation, the ground heat flux, and the sensible heat flux were treated as the independent variables. The model was tested with the data gathered during the campaigns and divided into a training period and a testing period. At a ratio of two-thirds of training to testing data, we achieved a Pearson correlation coefficient between the testing and predicted data of 0.66. The trained model was then applied to both years with the net radiation, the ground heat flux, and sensible heat flux as input parameters. As a last step, the predicted residual was subtracted from half-hourly ET. We assumed that the residual distributes equally to the LE and

Unlike for the methodological comparison and energy balance analysis, a gap-filling of

The residual energy was estimated from all available data in 2016 and 2017, following Eq. (

We trained the same machine learning tool as used for the ECEB set-up to predict the residual energy with the residual treated as the dependent variable and net radiation, ground heat flux, and sensible heat flux the independent variables.

The residual was predicted by the trained model; data gaps in the residuals, originating mainly from missing LE caused by data quality checks, were filled with the predicted values.

Subsequently, we distributed the residual to

The energy balance closure (EBC) was quantified in two ways:

First, as the linear regression between the sum of the turbulent flux components (

Second, as the energy balance ratio (EBR), also called the “instantaneous energy balance closure”

The spatial coverage and the position of the source area of turbulent sensible and latent heat fluxes and momentum at a specific point in time is defined by the flux footprint

The effects of structural differences between AF and MC on ET were studied in terms of the relationship between half-hourly ET and the aerodynamic and canopy resistances (

We studied the relationship between ET and canopy resistance and aerodynamic resistance for idealised ambient conditions with global radiation (

For the meteorological conditions during the campaigns, we refer to the time series of the relevant meteorological parameter in Fig.

Time series of daily mean air temperature (

Mean air temperature (

The flux footprint analyses showed that the measured turbulent fluxes were representative of the larger AF systems and their respective MC systems during the time of the experiments (e.g. Dornburg, Forst and Wendhausen, Fig.

Flux footprint climatologies for all sites for the respective campaign period. Green shaded footprints correspond to the AF system, and red shaded footprints correspond to the MC system. For the analysis only daytime data were used (

At the AF and the MC system of Wendhausen, we observed a 80 % flux magnitude contribution from both land uses to the total turbulent flux (Fig.

A total of 70 % of the area of the AF and MC grassland systems of Mariensee contributed to the measured fluxes, respectively (Fig.

The fluxes measured at the smallest AF system in Reiffenhausen were influenced by fluxes originating from the nearby forests and crop fields at about a 400 m distance from the flux tower in a northerly direction and about 200 m distance in a southerly direction (Fig.

The diel variation of ET for all three set-ups at all sites is depicted in time series plots for an exemplary time period in Fig.

The EC-LC set-up showed the best performance relative to direct EC measurements, with coefficients of determination between a minimum of 71 % and a maximum of 94 %. The EC-LC set-up captured the temporal variability of ET and the flux response to changing ambient conditions as well as the direct EC measurements. The slopes from a linear regression analysis of

At the MC systems of Forst and Wendhausen (Fig.

Time series of half-hourly evapotranspiration rates of an exemplary time period for ECEB, EC-LC, and EC as a reference for all sites. Time series of half-hourly ET rates for Reiffenhausen MC are missing due to the unavailability of a campaign, and

Statistical analysis results for a linear regression of

The mean EBC was

The EBC for

The EBC for

Scatterplot of the sum of the turbulent fluxes (

Statistical analysis results of the linear regression between the sum of the turbulent fluxes and the available energy, namely the sites, the set-up used, the slope (

The diel cycle of the energy balance ratio from

The Dornburg site might be affected by the horizontal advection of moisture and heat.

In addition to advective transport, the unclosed surface energy balance could be related to energy storage terms such as biomass, the air, or photosynthesis

Interestingly, the diel pattern of the EBR from

The diel cycles of the EBRs and the residuals were similar for both EC-LC and EC set-ups (Fig.

Median diel cycle of the energy balance ratio (EBR) and diurnal cycle of the residual energy for the AF and the MC systems at all sites. LE and

Median diurnal cycle of the energy balance components for Dornburg AF and MC for the campaign times (see Table

Sums of evapotranspiration for all three methods, all sites, and campaign periods indicate higher sums of

Sums of uncorrected and not gap-filled half-hourly evapotranspiration for all three methods and all sites during the campaign periods. Sites are abbreviated by their first letter and are identified as being either AF (agroforestry) or MC (monoculture). Incomplete records with

The annual cycle of evapotranspiration across all sites and for the years 2016 and 2017 depicts the typical seasonal cycle of the highest ET during summer and the lowest ET during winter (Fig.

Weekly sum of half-hourly

Differences between the annual sums of ET for the two land uses, AF and MC, were in the range of a maximum of

For this purpose, we used the relationship between the evapotranspiration index (

The figure indicates, first, that plots with an ET index larger than one were water limited, corresponding to an radiative dryness index

With regards to the first finding, in 2016 the grassland site of Mariensee MC and Reiffenhausen AF and MC had an ET index larger than one. At those sites, the annual sum of ET was generally high relative to the annual sum of precipitation (Fig.

The AF system in Reiffenhausen is located on a gentle slope with no groundwater access, which we expect should promote run-off, contrary to the high ET index observed. But the ET measurements are affected by a poplar and willow SRC in the south-southeast and north-northwest directly within the flux footprint (see Sect.

The second finding gives evidence of a dependency of ET on the local climate. The years 2016 and 2017 correspond to a dry and a wet year, respectively. In Fig.

However, our results indicate that the effect of agroforestry on ET is small compared to differences between methods and differences between years with different precipitation regimes. We therefore reject the initial hypothesis that short rotation alley cropping agroforestry systems lead to higher water losses to the atmosphere via ET, compared to monoculture agriculture without trees.

Annual sums of

Annual sums of energy balance closure corrected actual evapotranspiration, ET (

We wanted to understand if the heterogeneity of the AF systems can explain the differences between half-hourly ET rates from AF relative to MC systems. We quantified the effect of surface heterogeneity on ET as per the relationship between half-hourly ET rates and aerodynamic and canopy resistances. Tree strips orientated perpendicularly to the prevailing wind direction significantly reduce the wind speed

Mean aerodynamic resistances (

The relationship between half-hourly evapotranspiration rates and the canopy resistance at the sites followed an exponential function (Fig.

In the current study, the differences between the annual sums of ET over AF and MC were small. Effects of AF on evapotranspiration rates are mostly attributed to a small region next to the tree strips

Half-hourly

As outlined in the previous section, differences in annual sums of ET between the different land uses were small. Besides the discussed ecological reasons, we are aware of measurement errors due to the heterogeneous terrain

For the low-cost eddy covariance set-up we anticipate higher errors compared to direct EC due to the limited time response of the thermohygrometer and, subsequently, higher spectral correction factors

In contrast,

Although errors for ET measurements with the respective set-ups can be large on a half-hourly timescale, for annual sums of ET the errors often compensate each other and are small relative to the measured signal

The main objective of the current work was to investigate the effect of AF on evapotranspiration in comparison to monoculture agriculture without trees. We performed evapotranspiration measurements at multiple sites, for 2 consecutive years, with a low-cost eddy covariance set-up and an eddy covariance energy balance set-up.

In the first part of this paper, we investigated the performance of the measurement set-ups. In comparison with direct eddy covariance measurements, the low-cost eddy covariance set-up captured the temporal variability in half-hourly ET rates with high coefficients of determination during a comparison measuring campaign. The ECEB set-up also represented the diel cycle of ET but was characterised by more scatter. We therefore conclude that the EC-LC set-up is a viable alternative compared to conventional eddy covariance set-ups, as this set-up represents the ET of the underlying ecosystem more accurately than the ECEB set-up.

In the second part of the paper, we focused on the question of whether AF systems have higher water losses to the atmosphere via ET compared to monoculture systems. Our results showed that differences in ET between AF and MC were small. Instead, we found higher evapotranspiration indices during a drier than normal year compared to a wet year across sites and methods. This shows that the potentially small effect from the trees on ET was overlaid by the effect of local climatic conditions. In addition, we found a similar plant physiological response to the AF and the MC systems which is characterised by small differences between canopy resistances.

Overall, we conclude that the inclusion of tree strips into the agricultural landscape has not resulted in higher water losses to the atmosphere via ET, and agroforestry can be a land use alternative to monoculture agriculture without trees.

Half-hourly evapotranspiration rates in units of

The soil heat storage term has a major contribution to the unclosed energy balance

The soil heat storage (second term on the right-hand side of Eq.

The derivation of the water vapour mole fraction (

The water vapour mole fraction was derived from the definition of the specific humidity (

We then replaced the density of water vapour and the density of dry air in Eq. (

Solving Eq. (

The specific humidity in Eq. (

The actual vapour pressure (

The Penman–Monteith equation for the latent heat flux of a canopy

The slope of the saturation vapour pressure curve (

Rearranging Eq. (

The aerodynamic conductance for heat is as follows:

Temporal extent of the EC measurement campaigns.

Site-specific soil characteristics, with the soil texture being representative for the top soil column of 0.3 m. The bulk density is representative for the top soil column of 0.05 m. Data provided by

Flux footprint climatology for all sites and all available data during the years 2016 and 2017. Green shaded footprints correspond to the agroforestry system, and red shaded footprints correspond to the monoculture system. For the analysis only daytime data were used (

Scatter plot of

Scatter plot of

Median diel cycle of the energy balance ratio (EBR), and the diurnal cycle of the residual energy for the AF and the MC systems at all sites. LE was obtained by EC-LC. Data from Mariensee AF are from 23 March to 20 November 2016, and at Reiffenhausen MC the analyses are based on data collected from 7 April to 31 December 2016 because no data were available during the campaigns.

Bar plot of the evapotranspiration index for the ECEB method for the years 2016

All data used in this study are available at

CM designed and performed the field work, analysed the data, and wrote the paper. AK and LS wrote the project's scientific proposal, acquired the funding as part of the BonaRes SIGNAL consortium, and contributed to the field work and analysis. All authors contributed to the discussion and writing of the paper.

The authors declare that they have no conflict of interest.

We wish to acknowledge the contributions by Mathias Herbst to the BonaRes SIGNAL proposal and project design and the technical support through field work received from Frank Tiedemann, Edgar Tunsch, Dietmar Fellert, Martin Lindenberg, Johann Peters (Bioclimatology group), and Dirk Böttger (Soil Science group of Tropical and Subtropical Ecosystems) from the University of Göttingen.

This research has been supported by the German Federal Ministry of Education and Research (BMBF; project BonaRes, Modul A: SIGNAL; grant no: 031A562A) and the Deutsche Forschungsgemeinschaft (grant no. INST 186/1118-1 FUGG).

This paper was edited by Ivonne Trebs and reviewed by two anonymous referees.