Scope

Rfactor calculates rainfall erosivity from timestamped precipitation records.

The package implements a workflow for:

1. identifying independent rainfall events;
2. calculating maximum continuous rainfall intensities;
3. calculating rainfall kinetic energy;
4. calculating event EI30 erosivity;
5. classifying events according to configurable omission criteria;
6. aggregating contributing event EI30 values by calendar month or year;
7. calculating multi-year mean monthly or annual rainfall erosivity from those period totals.

The package does not calculate the remaining USLE or RUSLE factors and does not perform spatial interpolation or mapping.

The rainfall-event and erosivity methods implemented in Rfactor were empirically compared with RIST 3.99.10 using controlled synthetic rainfall records and real precipitation records.

RIST (Rainfall Intensity Summarization Tool) is a rainfall-analysis program developed and distributed by the U.S. Department of Agriculture, Agricultural Research Service (USDA ARS). The official RIST software page is maintained by USDA ARS.

Unless explicitly stated otherwise, statements about RIST behaviour in this vignette refer specifically to the RIST 3.99.10 fixed-interval mode used during package validation. They should not be interpreted as claims about every RIST version or input mode.

Event EI30 and the rainfall-runoff erosivity factor

For an individual rainfall event, Rfactor calculates EI30 as

[ EI_{30} = E I_{30}, ]

where:

• (E) is total event rainfall kinetic energy in MJ/ha;
• (I_{30}) is the maximum continuous 30-minute rainfall intensity in mm/h.

Event EI30 therefore has units of

[ . ]

The rainfall-runoff erosivity factor used by USLE and RUSLE is a long-term climatic quantity derived from erosive rainfall events over an appropriate historical record.

rf_calculate_rfactor() does not automatically estimate that climatological long-term mean annual quantity. It sums contributing event EI30 values within a requested calendar month or year.

Consequently, a yearly result from rf_calculate_rfactor() is the erosivity calculated from the events available for that year.

Multi-year mean monthly or annual rainfall erosivity can subsequently be calculated from these period totals with rf_calculate_mean_rfactor().

The averaging operation does not determine whether the underlying rainfall record is climatologically complete or representative. A representative long-term R-factor therefore still requires an appropriate multi-year record and a separate assessment of completeness and representativeness.

Rainfall-event separation

Rainfall events are identified from observations containing positive rainfall.

For the fixed-interval behaviour validated against RIST 3.99.10, consecutive positive-rainfall observations belong to the same event when their elapsed separation is less than or equal to the configured storm-break duration.

A new event begins when

[ t > t_{}. ]

With the default

[ t_{} = 6 , ]

the rule is therefore:

• exactly 6 hours: same event;
• more than 6 hours: new event.

Any positive rainfall observation resets the storm-break clock.

The storm-break duration is configurable through storm_break_hours.

Sparse rainfall records

Rfactor does not require a rainfall source file to contain every expected time step.

This is important for historical pluviograph records and timestamped rainfall archives in which dry periods may be absent.

The input reader preserves the observations supplied by the user. It does not create a complete rainfall series over the whole period of record.

Event identification operates from the timestamps containing positive rainfall. Once an event has been identified, a regular time grid is reconstructed only between its first and last positive-rainfall observations.

Expected positions absent from the supplied source record are assigned zero recorded rainfall for the calculation.

This treatment preserves elapsed time for rolling-intensity and kinetic-energy calculations without generating long sequences of explicit zero-rainfall rows between independent events.

An inserted zero does not assert that the corresponding interval was actually observed and dry. It is the computational representation used for a timestamp position absent from the supplied record.

The package does not estimate missing precipitation and does not assess whether the rainfall archive is climatologically complete.

Completely absent months or years are not created or imputed.

Maximum rainfall intensity

rf_max_intensity() calculates continuous rolling maximum rainfall intensity.

For a requested duration (D),

[ I_D = P_{D,} , ]

where (P_{D,}) is the greatest precipitation depth found in a continuous rolling window of duration (D), expressed in minutes.

Windows are not restricted to fixed clock boundaries.

For example, a 30-minute rainfall period beginning at 12:20 is evaluated as a complete 30-minute window even though it crosses the conventional 12:30 clock boundary.

Temporal-resolution compatibility

A requested rainfall-intensity duration must be an exact multiple of the rainfall observation interval.

Examples include:

EI30 requires a genuine 30-minute intensity. The rainfall interval must therefore divide 30 minutes exactly.

Data coarser than 30 minutes cannot resolve a genuine I30.

This also means that it would be incorrect to describe Rfactor as supporting every possible temporal resolution from 1 through 30 minutes. Validation was performed specifically at 1, 5, 10, 15, and 30 minutes.

Rainfall kinetic energy

For each fixed rainfall interval, precipitation depth is converted to rainfall intensity:

[ i_k = P_k , ]

where:

• (P_k) is interval rainfall depth in mm;
• (t) is interval duration in minutes;
• (i_k) is rainfall intensity in mm/h.

Unit rainfall energy (e_k) is calculated from the selected kinetic-energy equation in MJ/ha/mm.

Total event kinetic energy is

[ E = _k e_k P_k. ]

Rfactor provides three kinetic-energy equations.

"brown_foster_1987"

"mcgregor_1995"

"laws_parsons_1943"

Energy-equation validation

Three synthetic 30-minute constant-intensity events were evaluated at:

• 6 mm/h;
• 60 mm/h;
• 120 mm/h.

All omission criteria were disabled so that the kinetic-energy calculation could be compared independently.

The selected results were:

The 120-mm/h Laws–Parsons result was particularly important because it demonstrated empirically that the tested RIST calculation did not use the previously suspected 76.2-mm/h cap.

Event omission

Rainfall-event omission is configurable.

The default precipitation condition is

[ P < 12.70  ]

and the default intensity condition is

[ I_{15} < 25.40 . ]

Both criteria are enabled by default and

omit_logic = "all"

means that an event is omitted only when both enabled conditions are satisfied.

Thus, under the default configuration:

[ = (P < 12.70) (I_{15} < 25.40). ]

The inequalities are strict.

Consequently:

• (P = 12.70) mm does not satisfy the precipitation condition;
• (I_{15} = 25.40) mm/h does not satisfy the intensity condition.

The package uses a small floating-point tolerance when making these strict comparisons so that numerical representation error near an exact threshold does not change event classification.

omit_logic = "any"

omit_precip = FALSE
omit_intensity = FALSE

Single-record rainfall events

A dedicated edge-case experiment showed that RIST 3.99.10 assigned:

[ E = 0 ]

and

[ EI_{30} = 0 ]

to rainfall events represented by only one fixed-interval rainfall record.

Rfactor makes this behaviour optional.

The default is:

single_record_energy = "calculate"

which applies the selected energy equation normally.

To reproduce the observed RIST 3.99.10 behaviour, use:

single_record_energy = "rist_zero"

This is the principal reason that the complete set of Rfactor defaults should not be described as exactly identical to RIST defaults.

Canonical synthetic validation

A synthetic continuous 1-minute rainfall record was constructed to test the principal methodological rules against RIST.

The cases included:

• a 30-minute uniform event;
• an event crossing a conventional half-hour clock boundary;
• an event shorter than the 30-minute intensity window;
• a small low-intensity omitted event;
• a small high-intensity retained event;
• precipitation exactly equal to 12.70 mm;
• I15 exactly equal to 25.40 mm/h;
• rainfall separated by more than six hours;
• small positive intermediate rainfall resetting the storm-break clock;
• exact storm-break boundary cases;
• single-record events.

In the canonical comparison, Rfactor and RIST identified the same events and the same six erosive events.

For the complete synthetic comparison:

Differences shown here are consistent with the numerical precision reported in the RIST output.

Temporal-resolution validation

A second synthetic experiment tested fixed rainfall intervals of:

[ 1, 5, 10, 15, 30 . ]

The same underlying 1-minute rainfall record was aggregated before being processed independently by Rfactor and RIST.

Two 60-minute events were used:

1. a constant 6-mm/h event with total precipitation of 6 mm;
2. a variable-intensity event with total precipitation of 36 mm.

The variable event contained four consecutive 15-minute intensity periods of:

[ 12, 60, 24, 48 . ]

Temporal aggregation intentionally changed its I30.

Validation with real rainfall records

Synthetic experiments isolate individual algorithmic decisions, but the package was also tested with real precipitation archives from Meteo Romania (National Meteorological Administration).

The original Meteo Romania precipitation records are private and are not distributed with the package or public source repository.

Monthly and yearly aggregation

rf_calculate_rfactor() uses all supplied event rows to establish which calendar periods are represented.

Only events satisfying

erosive == TRUE

and having a non-missing EI30 value contribute to R and to n_events.

A represented month containing rainfall events but no contributing erosive events is returned as

R = 0
n_events = 0

A month or year containing no event rows at all is not generated.

Events are assigned to a calendar period according to event_start. An event crossing a month or year boundary is therefore assigned in full to the period in which it began.

Multi-year mean rainfall erosivity

rf_calculate_mean_rfactor() operates on monthly or yearly R-factor values, typically produced by rf_calculate_rfactor().

For yearly input, the multi-year mean is

[ {R} = _{y=1}^{n} R_y, ]

where (R_y) is an available yearly rainfall-erosivity value and (n) is the number of non-missing yearly values.

For monthly input, a separate mean is calculated for each calendar month:

[ {R}m = {y=1}^{n_m} R_{y,m}, ]

where (R_{y,m}) is the rainfall erosivity for calendar month (m) in year (y), and (n_m) is the number of non-missing values available for that month.

Missing R values are excluded from the calculation. Genuine zero values are retained because R = 0 represents an available period with no contributing erosive events.

For example,

year    R
2020  800
2021  900
2022   NA
2023  700
2024    0

produces

mean_R = 600
n_years = 4

because the missing value is excluded but the genuine zero remains in the calculation.

For monthly input, n_years is calculated separately for each calendar month. Different months can therefore have different numbers of contributing years.

Completely absent years or month-year combinations are not generated or imputed. A represented calendar month for which all supplied R-factor values are missing is returned with mean_R = NA and n_years = 0.

The mean annual R-factor is calculated directly from yearly R-factor values. It is not calculated by summing the twelve multi-year monthly means. Summing monthly means can give a different result when data availability differs among calendar months.

The function performs arithmetic averaging only. It does not assess whether individual monthly or yearly values were derived from complete rainfall records and does not determine whether the resulting mean is climatologically representative.

These averaging rules are protected by the package test suite. They are not derived from, or intended to reproduce, a separate RIST averaging algorithm.

What validation does and does not establish

The validation performed for Rfactor provides empirical evidence that the implemented methods reproduce the intended rainfall-erosivity calculations and closely reproduce RIST 3.99.10 for the tested fixed-interval cases.

It does not establish that:

• every version of RIST behaves identically;
• variable-interval RIST input behaves identically to the validated fixed-interval mode;
• every possible rainfall temporal resolution has been tested;
• a gapped rainfall archive is climatologically complete;
• monthly or yearly values calculated from incomplete records are representative climatological R-factor estimates;
• a multi-year arithmetic mean is necessarily a representative climatic R-factor when the underlying record is incomplete or unrepresentative;
• other USLE or RUSLE factors can be derived from Rfactor outputs.

Scientific interpretation of the resulting erosivity values therefore remains dependent on the quality, temporal resolution, completeness, and representativeness of the rainfall observations supplied by the user.

Reproducibility

The public source repository contains synthetic validation scripts and their generated comparison inputs under data-raw/.

These include dedicated experiments for:

• canonical RIST comparison;
• storm-separation edge cases;
• storm-break precipitation behaviour;
• kinetic-energy equations;
• temporal resolution.

The real Meteo Romania source precipitation files and their detailed real-data validation outputs are not distributed.

The package test suite additionally protects the numerical behaviour of the public functions, including strict omission boundaries, configurable storm-break duration, energy equations, temporal-resolution compatibility, sparse-event handling, period aggregation, and multi-year averaging rules.

References

Brown, L. C., and G. R. Foster. 1987. Storm erosivity using idealized intensity distributions. Transactions of the ASAE 30(2): 379-386. doi: 10.13031/2013.31957.

Laws, J. O., and D. A. Parsons. 1943. The relation of raindrop-size to intensity. Transactions, American Geophysical Union 24(2): 452-460. doi: 10.1029/TR024i002p00452.

McGregor, K. C., R. L. Bingner, A. J. Bowie, and G. R. Foster. 1995. Erosivity index values for northern Mississippi. Transactions of the ASAE 38(4): 1039-1047. doi: 10.13031/2013.27921.

Renard, K. G., G. R. Foster, G. A. Weesies, D. K. McCool, and D. C. Yoder, coordinators. 1997. Predicting Soil Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE). U.S. Department of Agriculture, Agriculture Handbook No. 703.

U.S. Department of Agriculture, Agricultural Research Service. RIST: Rainfall Intensity Summarization Tool. Available at: https://www.ars.usda.gov/southeast-area/oxford-ms/national-sedimentation-laboratory/watershed-physical-processes-research/research/rist/rist-rainfall-intensity-summarization-tool/.

U.S. Department of Agriculture, Agricultural Research Service. Using RIST to Convert Rainfall Records into Erosion Model Inputs. Available at: https://www.ars.usda.gov/southeast-area/oxford-ms/national-sedimentation-laboratory/water-quality-and-ecology-research/docs/using-rist-to-convert-rainfall-records-into-erosion-model-inputs/.

Wischmeier, W. H., and D. D. Smith. 1978. Predicting Rainfall Erosion Losses: A Guide to Conservation Planning. U.S. Department of Agriculture, Agriculture Handbook No. 537.