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 = 1,
starting_month = 1,
rep = 1,
keep_last = FALSE,
distribute = FALSE,
method = c("equal", "random", "gaussian"),
k = 10
)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
numeric, available water-holding capacity (mm), typically thickness (mm) * awc (fraction)
numeric, time-series of monthly PPT (mm), calendar year ordering
numeric, time-series of monthly PET (mm), calendar year ordering
numeric, initial fraction of AWC filled with water (values 0-1)
integer, starting month index, 1=January, 9=September
integer, number of cycles to run water balance
logical, keep only the last iteration of the water balance
logical, distribute monthly data into k divisions within each month
method for distributing PPT and PET into k divisions:
'equal' divides PPT and PET into k equal amounts
'random' divides PPT and PET into random proportions generated via multinominal simulation
'gaussian' divides PPT and PET according to a bell-shaped curve centered in the middle of each month
integer, number of divisions
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
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.