SWIB

1. About SWIB

SWIB (Soil Water Stress, Irrigation Requirement and Water Balance) is an online model for estimating daily plant/soil water stress (deficit or excess), crop irrigation requirements (and their impact of irrigation on aquifer storage) and water balance components based on measured daily soil moisture and precipitation. SWIB has been developed through a collaborative research effort between Canadian Rivers Institute (CRI), University of New Brunswick (UNB), Agriculture and Agri-Food Canada (AAFC) and Environment and Climate Change Canada (ECCC). SWIB is a result of a larger research effort aimed at evaluating the effects of agricultural production systems on groundwater and surface water quantity and quality. SWIB is part of the Hydrology Tool Set (HTS; https://www.hydrotools.tech), a suite of tools that can be used for advancing the understanding of various local and watershed scale hydrological processes. HTS also includes SepHydro (surface runoff and groundwater contributions to streamflow via hydrograph separation), ETCalc (potential, reference and actual evapotranspiration), SNOSWAB (water balance - snowfall, snowmelt, snowpack, soil water content, evapotranspiration, drainage, infiltration, surface runoff, etc.), RECHARGE BUDDY (groundwater recharge), and SNOWFALL BUDDY (snowfall and rainfall amounts).

If you have employed SWIB, we are kindly asking for the following citations to be included in your publication:

Danielescu S (2022) SWIB - An Online Model to Estimate Daily Crop Water Stress, Irrigation Needs, and Soil Water Budget. Groundwater.
DOI: https://doi.org/10.1111/gwat.13278.

Danielescu S, MacQuarrie K, Zebarth B, Nyiraneza J, Grimmett M, Levesque M (2022) Crop water deficit and supplemental irrigation requirements for potato production in a temperate humid region (Prince Edward Island, Canada). Water 14, 2748.
DOI: https://doi.org/10.3390/w14172748.

Danielescu S (2021) SWIB (Soil Water Stress, Irrigation Requirement and Water Balance) - A web based tool. Reference Manual.
Available at https://swib.hydrotools.tech.

2. Background

The amount of water stored in the soil is the resultant of the interdependence of soil, climate and biota. In addition, management related factors can be an additional driving force for soil water storage for ecosystems that are under significant human impact. For example, for agroecosystems, the irrigation regime can play a significant role on the amount of water stored in the soil during the growing season. Hence, knowledge of the state and dynamics of soil water storage is a key step for advancing our understanding relative to hydrological cycle, ecosystem health or agroecosystem productivity. In particular, soil moisture is a key element for many applications related for example to the impact of severe weather (e.g. droughts) on soil water availability; the impact of water stress (water deficit and excess) on natural vegetation and agricultural crops; the impact of climate change on soil water storage and vegetation; movement of water and solutes (e.g. contaminants) through soil; irrigation requirements for agricultural production; relationship between irrigation water supply and aquifer storage. Other examples of SWIB applications include the evaluation of the significance of soil properties on the dynamics and magnitude of crop water stress (either as water deficit or water excess), impact of irrigation triggering thresholds and scheduling on SWC, effect of irrigation losses on SWC and irrigation water supply requirements, influence of groundwater-sourced irrigation on aquifer storage as well as the evaluation of the various soil water budget components (e.g. evaporative losses, soil moisture losses and gains, soil drainage, infiltration and surface runoff). Broader applications of SWIB include generation of critical data for external models that allow for uploading of user-provided timeseries; or use in educational settings for example to demonstrate the significance of various of concepts related to crop water stress (i.e., water deficit or excess), irrigation practices and soil water balance.

SWIB online model fills an existing gap, by providing unrestricted access to a streamlined and robust framework, which uses a minimal amount of data to estimate daily water deficit and excess, irrigation requirements and impacts of irrigation on aquifer storage as well as several water balance components. SWIB methodology can be applied to any location as long as the required data is provided (e.g. minimal soil properties, daily soil water content and precipitation). Required soil properties (i.e. soil thickness and field capacity) are available either from direct field measurements or alternative sources such as soil surveys and technical guides. Daily soil moisture (i.e. soil water content) data is relatively easy to obtain for example from direct field measurements or from various publicly available databases. Daily precipitation is widely available from various weather monitoring stations. SWIB can be used for testing various scenarios even when field specific critical data (e.g. field capacity) is not available. In addition, the sample data set (i.e. test data) provided allows the user to experiment with the various modules of the model.

SWIB (Soil Water Stress, Irrigation Requirement and Water Balance) integrates three modules: 1) Water Stress (i.e. as water deficit or water excess); 2) Irrigation (and its impact on Aquifer Storage; and 3) Water Balance. The Water Stress module allows for calculation of the number of days with water deficit or water excess as well as of the daily amount of water deficit and/or excess relative to various soil moisture thresholds. The Irrigation module, allows for estimation of the crop irrigation needs, defining an irrigation schedule and calculating the impacts of irrigation on groundwater storage, assuming that groundwater is the main source for the irrigation water. The Water Balance module allows for estimation of several water balance components (e.g. soil water storage gain and loss, infiltration, surface runoff, evapotranspiration above the ground or in the soil) based on soil moisture data provided above and minimal weather data (i.e. precipitation, evapotranspiration). The web-based model provides various output and data visualization options through a streamlined process and a user-friendly interface.

3. Methodology

Details regarding data management as well as the methodology used for each of the three modules included in SWIB are presented below.

3.1. Requirements and formatting for input and validation data

Input data is a daily dataset consisting of:

• Soil moisture (SM). This is a required parameter for the Water Stress, Irrigation and Water Balance modules. SM is required for most of the parameters calculated by the model. SM, represents the Soil Water Content (SWC) and is equal to the soil Volumetric Water Content (VWC). SM is available from in situ measurements or from other sources (e.g. weather stations, online databases, etc.). SM can also be estimated using SNOSWAB (Snow, Soil Water and Water Balance Model; https://snoswab.hydrotools.tech), another tool included in the Hydrology Tool Set (HTS). In the input data file, the units for SM (VWC) are m³/m³;

• Total precipitation (TP). This is a required parameter for estimating the precipitation deficit (i.e. difference between precipitation and evapotranspiration) in the Irrigation module and the surface runoff in the Water Balance module. TP is the daily total precipitation and includes all forms of precipitation (e.g. snow, rain, freezing rain, etc.). TP is available from in situ measurements or from other sources (e.g. weather stations, online databases, etc.). The units for TP are mm;

• Evapotranspiration (ET). This is a required parameter for estimating the precipitation deficit (i.e. difference between precipitation and evapotranspiration) and the irrigation water supply in the Irrigation module and evapotranspiration above ground, evapotranspiration in the soil and infiltration the in the Water Balance module. ET is equal to the actual evapotranspiration. Although not directly calculated in SWIB, it can be estimated using ETCALC (https://www.etcalc.hydrotools.tech) or obtained via in situ measurements or other sources (e.g. weather stations, online databases, etc.). The units for ET are mm;

• Groundwater Recharge (RCH). This is an optional parameter used in the Irrigation module. RCH is equivalent to the net daily groundwater recharge, which is defined as the sum of the daily inflows into the aquifer on the days when the water table elevation of the aquifer is higher than the water table elevation for the previous day. RCH values are used in SWIB for calculating the net impact of irrigation on aquifer storage. RCH can be obtained from various sources (e.g. online databases) or can be estimated using RECHARGE BUDDY, another tool included in the Hydrology Tool Set (https://www.rbuddy.hydrotools.tech), which calculates RCH based on specific yield of the aquifer material and changes in water table elevation. If RCH values are not provided, SWIB will simply calculate the net effect of irrigation on aquifer storage (i.e. SWIB will consider the withdrawal of irrigation water from the aquifer and will ignore the RCH addition of water to aquifer storage). The units for RCH are mm.

Validation data is an optional data subset, and together with Input data forms the user input data file. SWIB allows for up to 5 validation datasets to be included in the user input data file. Validation data provides means for directly comparing any of the data output from SWIB with a dataset obtained using different methods or sources. The validation data set can include evapotranspiration data, soil moisture, soil temperature, groundwater table elevations, etc.

User input data file format: the user input data file has to be uploaded using a file with 1 column dedicated to calendar date, 4 columns dedicated to input data (TP - total precipitation, SM - soil moisture, ETA - actual evapotranspiration, RCH - groundwater recharge) and up to 5 columns dedicated to validation data (VD1 to VD5). The first four columns of the user input data consist of required data (i.e. Date, TP, SM, and ETA) and have to contain values in all rows. The rest of the columns in the user input file (i.e. RCH – groundwater recharge, VD1 to VD5 – validation dataset 1 to 5 are optional and can be left blank. SWIB allows uploading of files with maximum 7500 rows (~20 years of daily data). It is recommended to split the input data set in blocks of 20 years daily timeseries when the intent is to analyze longer time periods.

Notes:

• Required data:
  Date – use yyyy-mm-dd format;
  Total Precipitation [mm];
  Soil Water Content [m³/m³]
  Actual Evapotranspiration;

• Optional data:
  Groundwater recharge [mm];
  Validation data [user provided] - leave blank if no data is available

• The model requires daily data

• Use the first row of the data set for column headings

• SWIB includes limited input data quality check routines and hence, the user must ensure that the input data set is suitable for analysis (e.g., check dataset for missing values, parameters, erroneous values, etc.)

3.2. Time steps and averaging

SWIB uses daily data for both input and output data. SWIB includes a series of averaging options (i.e. monthly, seasonal [meteorological and growing season], yearly), which are explained in more detail below. Monthly frequency is recommended as the minimum time step for inspecting and analyzing SWIB results. SWIB can also calculate all the output parameters using a “typical year daily” (i.e. daily multi-year averages for each day of the year available in the input file) for the cases when the user input file contains data for more than one year. The monthly as well as the typical daily year (see below for details) are recommended for inspecting and analyzing results when multiple years of data are available. The following time intervals are available in tables, graphs and export files for displaying SWIB results:

• Daily: this is the operating time step for SWIB;

• Monthly;

• Yearly;

• Meteorological seasons, with the following distribution: Spring: March 1 – May 31st; Summer: June 1st – August 31st; Fall: September 1st – November 30th; Winter: December 1st – February 28th of the following year;

• Growing season [GS] and outside of growing season [OGS], based on the starting and ending dates specified by the user. OGS spans between the first day after the end of the growing season and the day before the start of the following year’s growing season;

• Typical year daily: one year of daily data based on the averages of the output data for each day in multiple years (i.e. 365 values) (e.g. for a 5 year daily data set, the value for Jan 1 will be the average for the values on Jan 1 in Year 1,2, 3, 4, and 5). This could be interpreted as the “representative daily year” or “average daily year”;

• Typical year monthly: one year of monthly averages (i.e. 12 values). The monthly averages are calculated using the typical year daily values;

• Typical year meteorological season: meteorological season averages (i.e. four values). The meteorological season averages are calculated using the typical year daily values;

• Typical Year growing season: growing season (GS) and outside of growing season (OGS) averages (i.e. two values). The growing season averages are calculated using the typical year daily values;

• Yearly: the average for all days in the typical year daily (i.e. one value).

3.3. Additional Parameters and Coefficients Required

• Input Data Module

  Start and end date for the growing season. Used for allowing SWIB to calculate all the parameters for the growing season (GS) and outside of the growing season (OGS). The start and end dates use mm-dd format (the year is ignored as the same dates are applied to all years available in the input data file).




• Water Stress Module

  Layer or Root zone thickness (mm). This is the thickness of the soil layer for which the analyses are conducted. Depending on the analysis it can be interpreted as the thickness of the soil profile, of a horizon or of the root zone. Layer or Root zone thickness is used for converting soil moisture (i.e., Volumetric Water Content) to equivalent height of water column (mm), to allow for calculation of the parameters in the Water Stress, Irrigation and Water Balance modules. The value has to be positive.

  Field capacity (m³/m³). Field capacity (FC) is typically equivalent to the water content of the soil at −33 kPa (i.e. −0.33 bar) matric potential. It should be noted that various reference values might be used, for example for sandy soils the -10 kPa (-0.1 bar) pressure is used for determining the field capacity. Field capacity represents the level of the soil moisture 2-3 days after an infiltration event has occurred (e.g. irrigation, snowmelt, rain, etc.), when most of the excess water has already drained from the soil.

  Water stress thresholds (T0, T1, T2, T3, T4). (% of FC). These thresholds are used for allowing the estimation of the severity of the water deficit, by calculating the number of days for which the soil is below (T1, T2, T3, T4) or above (T4) certain thresholds. The amount of water deficit or excess (e.g. mm of water) is also calculated for each of these thresholds. The thresholds represent percentages of field capacity (FC) and they can take values between 0 and 100. T0 and T4 are fixed thresholds, with T0 representing zero soil moisture (0% FC) and T4 representing the field capacity (100% FC). T1, T2 and T3 can be adjusted by the user. The percentage value has to increase from T1 to T4. All soil moisture values below T4 are considered to indicate soil water deficit, while soil moisture values above T4 are considered to indicate soil water excess. A scale bar at the bottom of the page shows the position of the various thresholds from a scale ranging from 0 to FC.




• Irrigation Module

  Start and end date of the irrigation period. Allows SWIB to trigger irrigation when SM is below the Irrigation Threshold during the Irrigation period. The format for the start and end dates is mm-dd (the year is ignored as the same dates are applied to all years available in the input data file);

  Irrigation threshold (%). During the irrigation period, the soil moisture threshold that triggers supplemental irrigation can be specified. This threshold is expressed as percentage of field capacity, and can have values between 0 and 100. Irrigation starts when soil moisture (SM) drops below this threshold and stops when the threshold is reached;

  Irrigation lost to ET (%). This coefficient allows SWIB to estimate the fraction of irrigation water that is lost to evapotranspiration. The irrigation water lost to evapotranspiration is one of the parameters that are included in calculating the irrigation water supply requirement. This coefficient is expressed as percentage of irrigation amount (i.e. irrigation requirement), and hence can have values between 0 and 100. Only the portion of the irrigation water that is left after the application of this coefficient is contributing to increasing soil moisture. This coefficient can also be interpreted as a more comprehensive coefficient for the overall irrigation efficiency, which would account for all the losses from the irrigation system (before the irrigation water enters the soil). As a result of applying this coefficient both the amount of irrigation water and actual evapotranspiration are recalculated. The recalculations impact only the Irrigation module and do not have an impact on the various components calculated with the Water Stress or Water Balance modules;

  Irrigation lost to groundwater (%). Allows calculation of the fraction of irrigation water that is returning to groundwater (i.e. return flow). The irrigation water lost to groundwater is one of the parameters that are included in calculating the irrigation water supply requirement. This coefficient is expressed as percentage of irrigation amount (i.e. irrigation requirement), and hence can have values between 0 and 100. As a result of applying this coefficient both the amount of irrigation water and the amount of (net) aquifer storage is recalculated. The recalculations impact only the Irrigation module and do not have an impact on the various components calculated with the Water Balance or Water Stress modules.




• Water Balance Module

  ET Above Soil (%). Allows calculation of the above ground and soil fractions of evapotranspiration. ET Above Soil is expressed as percentage (i.e. %) of evapotranspiration.

3.4. Water Stress Module

Soil water stress, calculated both as water deficit and water excess is based on the daily changes in soil moisture (SM). Based on the difference between soil moisture (i.e. VWC) and field capacity (FC) SWIB calculates if there is water deficit (VWC < FC) or water excess (FC < VWC) in the soil. In addition, several moisture thresholds, as specified by the user, are available for identifying the severity of the water deficit. The number of days with soil moisture below each of the thresholds (i.e. water deficit) as well as above the field capacity (i.e. water excess) and the average difference between these thresholds and VWC are calculated for each day. Independent from soil moisture changes, the precipitation deficit (also known as water deficiency) based on the difference between evapotranspiration (ET) and total precipitation (TP) is also calculated. A positive value indicates precipitation deficit, while a zero value indicates the absence of the precipitation deficit. Precipitation deficit can be used to as an alternative method for assessing crop irrigation requirements, however it does not account for soil moisture or soil properties, hence the soil moisture based method is preferred. The precipitation deficit method is prone to significant errors when calculated on the daily basis and hence, in SWIB this is calculated using on a monthly basis. The monthly averages are distributed to each day in the month in order to obtain a “simulated” daily time series for this type of deficit. However, it should be noted that the water deficit based on soil moisture represents an average (e.g. monthly average), whereas the water deficit based on the difference between precipitation and evapotranspiration is cumulative (e.g. monthly sum), and hence the two parameters are comparable only in terms of trends, but not as absolute quantities. Of note, precipitation deficit can show different dynamics and magnitude when compared with the more accurate, field-based soil moisture deficit method.

3.5. Irrigation Module

The conceptual model used for evaluating the irrigation requirements as well as the impact of irrigation on groundwater recharge is shown in the diagram below. SWIB calculates both the number of days when irrigation is required as well as the irrigation requirement (expressed as mm). The irrigation requirement is defined as the amount of water required to bring soil moisture at user-specified levels (i.e., irrigation thresholds) on days with water deficit (i.e., days with VWC < THR). In SWIB, the portion of irrigation lost to evapotranspiration is calculated based on a user provided coefficient, resulting in both an adjusted ET as well as an adjusted irrigation amount. Similarly, and considering that in SWIB the irrigation water is assumed to be sourced from groundwater, a portion of the irrigation water returns to groundwater (i.e., irrigation return flow). The significance of the return flow is controlled by a user-specified coefficient. The aquifer storage is adjusted following the addition of the return flow to the aquifer.

3.6. Water Balance Module

The conceptual model of SWIB Water Balance module is shown below.

Water balance is calculated using Eqn. 1. Actual evapotranspiration (ET; input data) is divided based on a user provided coefficient into evapotranspiration occurring above the ground due to for example canopy interception, ponding, etc. (ETAS) and evapotranspiration occurring in soil (ETFS). The daily soil moisture loss (i.e., Net loss) is calculated as the difference in measured soil water content between two consecutive days, when this difference is negative. The Net loss values for various averaging intervals is obtained by adding the daily net losses. Daily drainage (DR) is calculated as the difference between Net loss and the fraction of evapotranspiration occurring in the soil (ETFS), when this difference is positive and is considered to be absent when this difference is zero or negative. Soil is considered to gain moisture (i.e. Net gain) when the difference in measured soil water content between two consecutive days is positive. Infiltration and surface runoff can occur only on the days with Net gain in soil water storage. Infiltration (INF) is equal to the sum of daily Net gain and the fraction of evapotranspiration occurring in the soil profile. Surface runoff (SR) is calculated based on the difference between precipitation and infiltration. However, daily SR calculations could be subject to errors due to the delay between the time of precipitation and the actual response in soil moisture, as well as due to the possible presence of snowfall, snowpack and snowmelt during the colder periods of the year. Thus, the calculation of SR is based by default on a four-day window, with SR calculation triggered only for the windows when the average soil moisture content in the last two days of the window is larger than the average soil moisture content in the first two days of the same window (i.e. indicating that an infiltration event has occurred). The width of the window (i.e. number of days) for the calculation of surface runoff can be modified by the user. OTH represents losses or gains that are not accounted for and can also be interpreted as the budget error. OTH can be the result of the various assumptions used by the model as well as rounding up errors, and can be expressed as absolute value (mm; Eqn.1) or as percentage of total inputs (Eqn. 2).

All the calculations for the water balance are performed using millimetres of water (mm) units. Soil moisture (i.e. volumetric water content; VWC), which has units of m³/m³ in the user input file is converted to millimetres using the thickness of the soil specified by the user. The rates for the various water balance components (e.g., infiltration rate, drainage rate, etc.) can be calculated by dividing the values for the various components to the extent of the averaging interval.

3.7. Output Parameters

Below is a list off all coefficients and output parameters available in SWIB. These parameters are available via Table View and Export Data menu tabs.

5. Limitations

SWIB allows uploading of files with maximum 7500 rows (~20 years of daily data). It is recommended to split the input data set in blocks of 20 years daily timeseries when the intent is to analyze longer time periods.

SWIB includes limited input data quality check routines. Hence, the user is advised to conduct a thorough data quality check before uploading input data to minimize the risk for erroneous output.

Daily surface runoff calculations could be subject to errors due to the delay between the time of precipitation and the actual response in soil moisture (1-2 days) as well as due to the possible presence of snowfall, snowpack and snowmelt during the colder periods of the year.

6. Terms of Use

SWIB can be used freely.

The authors do not assume any responsibility for the model's operation, output, interpretation, or use of results.

7. Contact

Serban Danielescu, Ph.D.
Research Scientist | Chercheur scientifique
Environment and Climate Change Canada | Environnement et Changements Climatiques Canada
Agriculture and Agri-Food Canada | Agriculture et Agroalimentaire Canada
Fredericton Research and Development Centre | Centre de recherche et développement de Fredericton
95 Innovation Rd., Fredericton, NB, E3B 4Z7
Telephone/Téléphone: 506-460-4468
Facsimile/Télécopieur: 506-460-4377