swb
— Calculation of soil water balance¶

class
swb.
SoilWaterBalance
(fc, wp, rd, kc, p, precipitation, evapotranspiration, irrigation_efficiency, rd_factor=1)¶ Calculates soil water balance. The init parameters provide values for some of the attributes (see below). The heart of the class is the
root_zone_depletion()
method; see its documentation for the methodology used.
evapotranspiration
¶ A
Timeseries
object with a daily evapotranspiration series; provided at class initialization time.

fc
¶ The field capacity, provided at class initialization time.

irrigation_efficiency
¶ Irrigation method efficiency factor, provided at class initialization.

kc
¶ The crop coefficient, provided at class initialization time.

p
¶ The crop depletion fraction, i.e. RAW/TAW, provided at class initialization time.

precipitation
¶ A
Timeseries
object with a daily precipitation series; provided at class initialization time.

raw
¶ The readily available water:

rd
¶ The crop root depth, provided at class initialization time. It can be in any unit of length. If it is in a different unit than water depth variables (such as evapotranspiration, precipitation, irrigation and depletion)
rd_factor
is used to convert it.

rd_factor
¶ If the root depth is in a different unit than the water depth variables (such as evapotranspiration, precipitation, irrigation and depletion)
rd_factor
is used to convert it. If the root depth is in metres and the water depth variables are in mm, specifyrd_factor=1000
. Provided at class initialization time.

taw
¶ The total available water:

wp
¶ The wilting point, provided at class initialization time.

depletion_report
¶ A list with the intermediate calculations made by
root_zone_depletion()
. Before the first time the method is called, it is an empty list.

root_zone_depletion
(start_date, initial_soil_moisture, end_date)¶ This method calculates, in a simplified way, the root zone depletion. The basis for the calculation is this formula:
D_{r,i} = D_{r,i1}  (P_{i}  RO_{i})  IR_{n,i}  CR_{i} + ET_{c,i} + DP_{i}(FAO56, p. 170 eq. 85)
where:
 i is the current time period (i.e. the current day).
 D_{r,i} is the root zone depletion at the end of the previous time period.
 P_{i} is the precipitation.
 RO_{i} is the runoff.
 IR_{n,i} is the net irrigation depth.
 CR_{i} is the capillary rise.
 ET_{c,i} is the crop evapotranspiration.
 DP_{i} is the water loss through deep percolation.
with the following limits imposed on D_{r,i}:
0 <= D_{r,i} <=taw
(FAO56, p. 170 eq. 86)
RO_{i}, CR_{i} and DP_{i} are ignored and considered zero. The equation therefore becomes:
D_{r,i} = D_{r,i1}  P_{i}  IR_{n,i} + ET_{c,i}ET_{c,i} is calculated using crop coefficient approach by multiplying
evapotranspiration
by crop coefficientkc
.The essential simplifying assumption of this method is that each time we irrigate we reach field capacity (i.e. zero depletion). Therefore, at the last irrigation date we have i=1 and D_{r,1}=0. The equation then becomes:
D_{r,i} = D_{r,i1}  P_{i} + ET_{c,i}(we do not use IR_{n,i}, since, if we irrigated, according to our assumption, we would restart with i=1 and D_{r,1}=0).
The point i=1 is specified by start_date, which is a
datetime
object. The initial_soil_moisture will usually equalfc
(this, according to the essential simplifying assumption, means that the crop was irrigated on start_date). However, if the crop has not been irrigated recently, initial_soil_moisture will be set to another value (such as a soil moisture measurement made at start_date).Soil moisture and depletion are related with this formula:
moisture = fc  depletion / (rd * rd_factor)(FAO56, p. 170 eq. 87)
so, since the initial_soil_moisture is given, D_{r,1} is also known.
The method returns the root zone depletion for end_date in millimeters (mm).
precipitation
andevaporation
must have nonnull records for all days from the day following start_date to end_date.

irrigation_water_amount
(start_date, initial_soil_moisture, end_date)¶ This method calculates irrigation water needs based on
root_zone_depletion()
andirrigation_efficiency
factor (i.e. drip, sprinkler).The method returns irrigation water needs for end_date in millimeters (mm).

References¶
R. G. Allen, L. S. Pereira, D. Raes, and M. Smith, Crop evapotranspiration  Guidelines for computing crop water requirements, FAO Irrigation and drainage paper no. 56, 1998.