TITLE: USDA LTAR Common Experiment measurement: Infiltration and percolation
AUTHORS: Amartya Saha

[DESCRIPTION]
Infiltration refers to water entering the soil from precipitation, condensation, irrigation, and surface flow (runoff). It is typically used to quantify water arriving at the root zone of plants (example application: plant water availability). Percolation, while verbally synonymous with infiltration, is typically used to quantify water leaving the root zone for deeper soil horizons (example application: agrochemical leachate). Both processes occur when the soil is near saturation, relying on the continuity of the water column for the downward movement of water.

[STEPS]
SECTION: Data collection
2. Measurement.
The amount of water collected is measured over a particular time interval of collection. 

 

SECTION: Data collection
2.1. Perform sampling periodically or follow an intense rain event (typically 2-3 cm of rainfall).
SECTION: Data collection
3. Site Maintenance. 
Lysimeters are typically installed underground with the top percolation collection area slightly underneath the root zone - typically 30 cm below the ground surface (but they can be installed at any depth depending on where leachate collection is needed or percolation is measured). 
SECTION: Data collection
3.1. Use a post-hole digger, auger or shovel can be used to excavate the hole, evenly filling in the excavated soil and avoiding preferential flow paths for percolating water. 

 

SECTION: Data collection
3.2. Enclose the data cables and pump access/flush tubes with an electrical PVC conduit to protect against rodents, accidental abrasion by shovels or mowers, and moisture.
SECTION: Data collection
3.3. Maintain vegetation cover directly above the lysimeter to resemble the vegetation cover in the area, especially if the lysimeter is placed in a fenced enclosure in a grazed rangeland (to prevent trampling by cattle or feral hogs) that promotes grass growth.
SECTION: Data processing and quality control
4. Automated lysimeters (such as the G3 Drain Gage) send real-time data to a data logger (such as Campbell Scientific or METER) that stores data in a text file.

Manually measured lysimeters necessitate some means for user data entry, such as a field notebook or phone-based app (example: ArcGIS Survey 123).

 

SECTION: Data processing and quality control
4.1. The presence of soil moisture data (if available at the same location or a nearby location) serve to verify saturation conditions (typically approximately 0.4-0.5 water by volume fraction, or 40-50%, depending on soil type) and thereby help examine whether zero values for percolation are legitimate or arising from instrument failure.

Because percolation mainly occurs as rain events in near-saturated soil conditions, it is common for values to be zero for long periods of the dry season as well as parts of the wet season.
SECTION: Data processing and quality control
4.2. Saturated soils facilitate percolation because of the continuity of hydraulic pathways downward in the soil profile; such conditions exist intermittently over the wet season. 

 

SECTION: Data file formats and metadata
5. Text files or user-created spreadsheets/databases. Ancillary data are rainfall and soil moisture at various depths above the lysimeter placement. For example, soil moisture sensors at 5, 10, 20, and 50 cm can indicate if a rain event (or irrigation) results in a wetting front percolating downward in the soil profile, which can be captured by a lysimeter placed at a depth of 30 cm. 

 

SECTION: Data file formats and metadata
6. Recommendations on data storage and metadata: Follow the general recommendations in the USDA LTAR Common Experiment measurement: Best practices for collection, handling, and analyses of water quantity measurements protocol (Baffaut, 2024). 
SECTION: Recommendations for data collection
7. Table 1. Summary of recommendations for measuring infiltration/percolation.

 

SECTION: Selecting a method
1. The specific method chosen depends on whether the objective is to estimate infiltration/percolation rates or the actual collection of leachate for chemical analysis. Note that the estimation of groundwater pollution from surface pollutant sources entails both objectives. An example of the former objective is a pan lysimeter located underground that collects water trickling down (when soil is near saturation). This amount of water collected over a period of time is expressed on the basis of weight or volume,  thus yielding an estimate of the infiltration rate at that soil depth/horizon. An example of the latter objective is a suction lysimeter that actively draws in and collects water from the unsaturated/vadose zone by means of a vacuum generated by pumping within the lysimeter. The leachate collected is subsequently analyzed for pollutants such as agrochemicals. An example of a device that can be used for both objectives is a passive-wicking lysimeter that collects water from the soil just above the lysimeter using a fiberglass wick. The dimensions of this wick are designed to passively overcome the capillary forces prevailing in soil. The water then trickles into a collection chamber from where it can be pumped out manually, or automatically by a pump triggered by a water level recorder.