Perform a monthly water balance by "leaky bucket" model, inspired by code from bucket.sim of hydromad package, as defined in Bai et al., (2009) (model "SMA_S1"). The plant available water-holding storage (soil thickness * awc) is used as the "bucket capacity". All water in excess of this capacity is lumped into a single "surplus" term.

monthlyWB(
  AWC,
  PPT,
  PET,
  S_init = AWC,
  starting_month = 1,
  rep = 1,
  keep_last = FALSE
)

monthlyWB_summary(w)

Arguments

AWC

available water-holding capacity (mm), typically thickness (mm) * awc (fraction)

PPT

time-series of monthly PPT (mm), calendar year ordering

PET

time-series of monthly PET (mm), calendar year ordering

S_init

initial fraction of AWC filled with water

starting_month

starting month index, 1=January, 9=September

rep

number of cycles to run water balance

keep_last

keep only the last iteration of the water balance

w

used for for monthlyWB_summary(): a data.frame, such as result of monthlyWB();

Value

a data.frame with the following elements:

  • PPT: monthly PPT (mm)

  • PET: monthly PET (mm)

  • U: monthly surplus (mm)

  • S: monthly soil moisture storage (mm)

  • ET: monthly AET (mm)

  • D: monthly deficit (mm)

  • month: month number

  • mo: month label

monthlyWB_summary(): a data.frame containing cumulative (dry, moist, wet) and consecutive (dry_con, moist_con, wet_con) number of days dry/moist/wet, total deficit (total_deficit), total surplus (total_surplus), total actual evapotranspiration (total_AET), and annual actual evapotranspiration to potential evapotranspiration ratio (annual_AET_PET_ratio)

Details

See the monthly water balance tutorial for further examples and discussion.

A number of important assumptions are made by this style of water balance modeling:

  • the concept of field capacity is built into the specified bucket size

  • the influence of aquitards or local terrain cannot be integrated into this model

  • interception is not used in this model

References

Arkley R, Ulrich R. 1962. The use of calculated actual and potential evapotranspiration for estimating potential plant growth. Hilgardia 32(10):443-469.

Bai, Y., T. Wagener, P. Reed (2009). A top-down framework for watershed model evaluation and selection under uncertainty. Environmental Modelling and Software 24(8), pp. 901-916.

Farmer, D., M. Sivapalan, Farmer, D. (2003). Climate, soil and vegetation controls upon the variability of water balance in temperate and semiarid landscapes: downward approach to water balance analysis. Water Resources Research 39(2), p 1035.