Skip to contents

Convert ETo to ETc using BIS/BISm crop coefficients

Source: R/eto_to_etc.R
eto_to_etc_bism.Rd

Convert grass-reference evapotranspiration (ETo) to crop evapotranspiration (ETc) using crop coefficient (Kc) schedules from the BIS/BISm framework (Snyder et al.). Daily ETc is calculated as ETc = Kc * ETo.

Kc values are determined using one of two mutually exclusive timing modes, selected by input:

  1. Date-based (percent-of-season) mode: If date is provided, Kc is interpolated linearly between BIS/BISm growth-stage anchors (B–C–D–E) using percent-of-season, with default planting and harvest dates taken from bism_kc_by_crop.

  2. Canopy-cover mode: If canopy_cover is provided, Kc is estimated from observed ground cover. For field and row crops, canopy cover is used to locate position within the B–C growth phase (linear increase from Kc_B at ~10% cover to Kc_C at ~75% cover, capped thereafter). For tree, vine, and subtropical crops, Kc is scaled from the mature mid-season value using the immaturity relationships of Snyder et al. (Snyder, Fig. 15).

Exactly one of date or canopy_cover must be supplied.

Usage

eto_to_etc_bism(eto, crop_name, date = NULL, canopy_cover = NULL)

Arguments

eto

Numeric vector of reference evapotranspiration (ETo).

crop_name

Character scalar identifying a crop in bism_kc_by_crop.

date

Optional Date vector (same length as eto); triggers percent-of-season interpolation using BIS/BISm defaults.

canopy_cover

Optional numeric vector giving fractional ground cover (0-1), length 1 or length(eto); triggers canopy-cover-based Kc estimation.

Value

Numeric vector of crop evapotranspiration (ETc), with the same units as eto (e.g., mm d-1).

References

Snyder, R., Orang, M., Bali, K., Eching, S., Zaccaria, D. (2014). BISm Basic Irrigation Scheduling Excel program (metric units). University of California Cooperative Extension.

Snyder, R.L., 2014. Irrigation scheduling and soil water budgeting (ISWBM). University of California, Davis. https://biomet.ucdavis.edu/basic-irrigation-scheduling-(BIS).html

Doorenbos, J., Pruitt, W.O. (1977). Guidelines for predicting crop water requirements. FAO Irrigation and Drainage Paper 24.