7.5 Subprogramme TF: Throughfall

• 7.5.1 Introduction
• 7.5.2 Sampling methods
• 7.5.2.1 Siting and number of collectors
• 7.5.2.2 Type of collector
• 7.5.2.3 Sampling frequency
• 7.5.2.4 Collection and handling of throughfall samples
• 7.5.3 Chemical analyses
• 7.5.4 Quality assurance/Quality control
• 7.5.5 Data reporting
• 7.5.6 References

7.5.1 Introduction

In forests part of the precipitation falls through gaps in the canopy without being intercepted and part is intercepted during its passage through the canopy. Together the parts are called (crown) throughfall. The part running down the tree trunk is called stemflow (see SF subprogramme). Together, throughfall and stemflow can be called total throughfall or stand precipitation. The main purpose of the TF subprogramme is to enable the total deposition input to the soil under the forest canopy and forest vegetation to be determined. Empirical methods have been used for the quantification of total deposition to forested ecosystems (Bredemeyer 1988). In forested areas, throughfall and bulk deposition from an open area (see subprogramme Precipitation chemistry, PC) are both needed to estimate the total deposition input to forested sites. This is done by comparing TF with PC, to assess canopy interception and the interaction and internal cycling of nutrients. For some types of forest stands, also stemflow (see Stemflow SF subprogramme) is needed.

7.5.2 Sampling methods

7.5.2.1 Siting and number of collectors

Besides the general siting criteria given in Chapter 5.2, those given in the ICP Forests manuals should be considered. Throughfall deposition measurements should be carried out in such a way that the other monitoring activities are not influenced significantly by the measurement procedure.

In order to take into account the large local variations in throughfall deposition, a sufficiently large number of collectors must be used. Less than 10 samplers are usually insufficient to cover the variability. Carry out a pre-study to assess the variability of the forest stand in relation to how many throughfall collectors are necessary to collect a sample representative for the forest in question.

The collectors can be sited randomly or systematically (recommended) around the vegetation or soil intensive monitoring plots or form their own station nearby. Disturbances to the TF collectors by large animals may be prevented by surrounding the TF collectors with a fence.

The throughfall collectors should be placed with the collection surface horizontal to the ground surface at a height of approximately 1 m to prevent contamination from the soil. It is important to shield the sample container from sunlight and warming. It is therefore recommended to store the sample containers in a cool and dark place e.g. in a pithole.

7.5.2.2 Type of collector

The TF collectors may be funnel or gutter types but must be constructed from a material which does not alter the chemical composition of the sample. The TF collectors should preferably be the same as used for the PC subprogramme. This is because the amount of precipitation collected and evaporation losses vary with the type and design of collector. If the same collectors are used for both TF and PC the collectors will have the same collection and evaporation efficiencies and that loss of water due to evaporation is compensated for by a corresponding increase in concentrations. For snow sampling special collectors are recommended. See also PC Chapter.

7.5.2.3 Sampling frequency

Sampling will be made monthly, weekly or at a time interval between the two, e.g. every two or three weeks, depending mainly on climate and method used. It is recommended that precipitation samples are taken so that correct monthly values can be derived. Long sampling period may to some extent cause biodegradation of the samples. By shielding the samplers using aluminium foil, this degradation will be strongly reduced. It is, however, not recommended to add preservatives.

The sampling integration time should preferably be the same for all deposition measurements (i.e. throughfall, stemflow and bulk deposition).

After each sampling period, the volume of each individual throughfall sample must be determined. Throughfall samples from a number of collectors may be pooled to a composite sample representative for a certain stand. Weekly samples can be analysed or mixed to monthly samples before analyses. If samples are mixed they must be mixed in proportion to the total sample volume. Special care must be taken in the mixing procedure in order to avoid contamination and errors.

7.5.2.4 Collection and handling of throughfall samples

The recommended methodology for sampling, sample handling and cleaning is described in detail by ICP Forests manual, part VI and by EMEP, Chapter 3.1.4 - 3.1.5. There are special demands for the trace metal sampling, recommendations on sampling of heavy metals are incorporated in the new EMP manual.

The general procedures for collection and handling of all water samples are described in Chapter 8.2.

7.5.3 Chemical analyses

The TF subprogramme consists of the following mandatory parameters:

sulphate, nitrate, ammonium, total N, chloride, sodium, potassium, calcium, magnesium, dissolved organic carbon and strong acid (by pH). It is also recommended to determine the electrical conductivity and to determine alkalinity in the samples if annual median pH>5. The determination of total S and heavy metals is optional.

The recommended method for determination of the major ions in precipitation samples is ion chromatography. Suitable alternative methods are for example atomic absorption spectrometry for Na, K, Ca, Mg and spectrometric methods for ammonium. The recommended method is described in the EMEP manual, Chapter 4.1, alternative methods are described in EMEP, Chapters 4.2 - 4.6. A list of available standards is given in Chapter 8.5.

Optional parameters:

The recommended method for the determination of total nitrogen is by oxidation to nitrate by peroxidisulphate following the standard ISO/DIS 11905-1 and analysis by the spectrophotometric Griess method (EMEP, Chapter 4.3). Alternatively, total nitrogen may be determined by the Kjeldahl method. A suitable method for the determination of total sulphur (optional) is inductively coupled plasma spectrometry (ICP) ISO - 11885.

The recommended method for determination of pH, strong and weak acids is potentiometry, as described in the EMEP manual, Chapter 4.7. An alternative method for the determination of strong and weak acids is the coulometric titration method (modified Gran_s titration). This method is described in the EMEP manual, Chapter 4.8.

It is recommended to determine alkalinity if annual median pH>5. The recommended method for determining alkalinity is described in the standard EN ISO 9963-1 or alternatively by colorimetric titration (see above).

The recommended method for determination of conductivity is conductometry. The method is described in detail in the EMEP manual, Chapter 4.9.

The EMEP has included recommendations on sampling and chemical analysis of heavy metals in the EMEP manual (see EMEP web-manual).

7.5.4 Quality assurance/Quality control

It is very important to have a good quality of data, both being consistent in time (in order to assess trends) and space (for the comparisons between different sites and countries). The general procedures for quality assurance given by EMEP, Chapter 3.1.8 as well as procedures in Chapter 8 of this manual should be followed. The QA/QC procedures should include all parts of the activities performed at the site, and in the laboratory.

Standard operation procedures should be followed for all activities. Necessary equipment, cleaning materials, sufficient supply of spare parts etc. must be available. All operators should be well trained and sites and equipment must be inspected/controlled at least once a year by the quality assurance manager/data originator. The QA/QC routines in the field include addition of field blanks and control samples, and also requirements for sample transportation.

It is expected that the chemical laboratory is accredited under one of the laboratory accreditation systems, or is performing close to these standards, e.g. EN 45001 and ISO/IEC guide 25. The laboratory must check on its performance, with respect to detection limits, precision and repeatability, by repeated analyses of control solutions etc.

It is strongly recommended to participate annually in international intercomparisons for all analysed compounds. It is also recommended to participate in field intercomparisons. The ICP IM Programme Centre will be able to give information about relevant intercalibrations. All data should be verified and validated following the instructions given by EMEP, Chapter 5 and 6.

The quality assurance programme described by ICP Forests manual, part VI should also be followed.

7.5.5 Data reporting

Example files

TF example Excel file
TF example ASCII file

• File identifier SUBPROG states the subprogramme.    
• MEDIUM refers to the dominating tree species (from NCC code list B4, see Annex 6) of the stand. If two species are equally dominant, the species reported as medium for throughfall is the one with the largest intercepting leaf area.    
• LEVEL is given as the distance of sampling devices from the ground (cm).    
• Spatial pool SPOOL refers to the number of individual samplers used for each parameter.
• Values from weekly measurements are reported as volume weighted monthly means, status flag is W. If the throughfall amount can not be adequately measured, the concentrations for weekly measurements are reported as monthly means, status flag is X. Monthly measurements are reported without status. Throughfall amount is reported as monthly sum, status flag is S.For calculation of volume weighted means, please see Annex 7. General information on flags is given in Chapter 4.
• Sampling year and month are given as YYYYMM, day field is left blank

Codes for most common European tree species from NCC code list B4 (see Annex 6):

ABIE ALB, Abies alba
ABIE NOR, Abies nordmanniana
ACER CAM, Acer campestre
ACER PLA, Acer platanoides
ACER PSE, Acer pseudoplatanus
ALNU GLU, Alnus glutinosa
ALNU INC, Alnus incana
BETU PEN, Betula pendula
BETU PUB, Betula pubescens
BE PU.TO, Betula pubescens ssp.tortuosa
CARP BET, Carpinus betulus
FAGU SYL, Fagus sylvatica
LARI DEC, Larix decidua
LARI SIB, Larix sibirica
PICE ABI, Picea abies
PI AB.OB, Picea abies ssp. obovata
PICE GLA, Picea glauca
PICE OMO, Picea omorika
PINU SYL, Pinus sylvestris
POPU BAL, Populus balsamifera
POPU NIG, Populus nigra
POPU TRA, Populus tremula
PRUN PAD, Prunus padus
QUER PET, Quercus petraea
QUER ROB, Quercus robur
TILI COR, Tilia cordata
TILI PLA, Tilia platyphyllos
ULMU GLA, Ulmus glabra
ULMU LAE, Ulmus laevis

7.5.6 References

EMEP web-manual: EMEP manual for sampling and analysis
http://www.nilu.no/projects/ccc/manual/

ICP Forests Manual, 2016
http://icp-forests.net/page/icp-forests-manual

ICP Forests manual, 1997. Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests, 4th edition. Edited in 1997 by the Programme Coordination Centre Federal Research Centre for Forestry and Forest Products (BFH), Hamburg, Germany.

ICP Forests manual, 1994. Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests. 1994 edition. Edited by the Programme Coordination Centres Hamburg and Prague.

Bredemeier, M. 1988. Forest canopy transformation of atmospheric deposition. Water, Air, and Soil Pollution 40:121-138.