Introduction

Evapotranspiration is one of the major components of Earth’s water balance, being the sum of evaporation and plant transpiration from the land and ocean surface. Factors that affect evapotranspiration includes solar radiation, wind, humidity, temperature, growth stage of vegetation, and water availability. The latter depends on factors such as precipitation, irrigation, and soil characteristics.

Local- or field-scale evapotranspiration can be measured by a weighing lysimeter, the energy balance Bowen-ratio method, or the eddy covariance technique, but these instruments and methods are expensive, may be susceptible to errors, or require correction (Healy, 2010). Alternatively, field-scale evapotranspiration can be estimated from climatic data by simulating the water balance of an area with a specific vegetation growing on a specific soil. This report documents a code (a package), programmed in Python and named Edcrop, which can do such local simulations for various types of soil and vegetation. It does not simulate surface flow, lateral flow, nor flow processes taking place in the saturated zone (e.g. loss of water to drains). The water balance equation of Edcrop is therefore simply:

\({P = E_a + D + ∆V}\)

where \(P\) is precipitation (possibly including irrigation), \(E_a\) is actual evapotranspiration, \(D\) is downward drainage to the unsaturated zone beyond the root zone, and \(∆V\) is change in water storage. Evapotranspiration includes evaporation of snow, evaporation of intercepted water, evaporation of soil water, and plant transpiration. \(∆V\) includes change in snow pack, change in intercepted water, and change in water content in the root zone and in the subzone. The subzone is the zone between the bottom of the root zone and the bottom of the model’s soil profile.

The conceptual model implemented in Edcrop is a modification of the Evacrop model by Olesen and Heidmann, 2002, which itself builds on the Watcros model by Aslyng and Hansen (1982). The Edcrop conceptualization is based on considerations regarding the physical processes that are important for turning precipitation and irrigation into either evaporation, transpiration, or drainage from the root zone: Temperature determines whether precipitation falls as rain or snow, and it determines when snow thaws and infiltrates. The vegetation intercepts a part of precipitation, while the rest infiltrates into the ground. The infiltrated water will either evaporate, be absorbed by plant roots, be stored in the soil, or drain from the root zone. Potential evaporation is distributed between vegetation and soil, where the former part drives evaporation of intercepted water and plant transpiration from the green leaf area, while the latter part drives evaporation from the soil. The soil’s ability to store water depends on its field capacity; when the water content exceeds field capacity, water will gradually drain downwards. Furthermore, it is assumed that the annual life cycle of crops and wetland vegetation is driven by growing degree-days only (and not by any other climatic variables), while for forests the life cycle is described by a calendar. For irrigation, either (i) date and amount are input, or (ii) they are determined automatically by Edcrop using certain criteria.

There are two alternative soil water balance functions to choose between in Edcrop. The first alternative is an almost straight copy of the function used in the original Evacrop code by Olesen and Heidmann, 2002, simulating flow through the soil profile as flow through two linear reservoirs using daily time steps. However, it can simulate macro-pore drainage, which the original Evacrop cannot. The second alternative simulates flow through the soil profile as flow through four linear or nonlinear reservoirs using daily or sub-daily time steps. For nonlinear reservoirs, Edcrop uses Mualem – van Genuchten like functions. It also simulates gravity driven macro-pore flow as well as loss of infiltration due to surface runoff.

As input, given in text files, Edcrop requires daily temperature, precipitation, and reference evapotranspiration. It also requires information about combination(s) of soil type and vegetation type to simulate. One can choose between seven default soil types and fifteen default vegetation types, or one can manually input information for other types of soil or vegetation. In a single model run, Edcrop can loop through lists of climate files, soils, and vegetation.

The seven default soil types vary from coarse sandy soil to clayey soil. The fifteen default vegetation types include bare soil, ten types of crop, two types of forest, and two types of wetland.

As said, the water balance simulation of Edcrop is similar to that of Evacrop (Olesen and Heidmann, 2002), but in other ways Edcrop is different from Evacrop. Edcrop allows more flexible and easier specification of input and output; it can loop through lists of climate, soil, and vegetation combinations in a single model run; it simulates macro-pore drainage; it contains more crops than Evacrop; it contains forest and wetland types, which are new compared to Evacrop; it has a more advanced irrigation module; and data and results can be plotted.

Edcrop cannot simulate capillary rise of shallow groundwater to the root zone or surface. If downward drainage (just called drainage in the following) simulated by Edcrop is to be used as recharge input for a groundwater model, there can be ways to partly correct for this lack of Edcrop ability. For example, for simulation of groundwater flow using Modflow (McDonald and Harbaugh, 1988), drainage from Edcrop can be used as recharge input for the Modflow RCH package, while the difference between Edcrop’s potential evapotranspiration and actual evapotranspiration can be used as maximum ET input for the Modflow EVT package. Similar can be done using newer versions of Modflow.