Figure 2: Top, from left to
right: Soil sampling locations within the Pinios Hydrologic Observatory,
soil data from the European soil database and the LUCAS topsoil map.
Bottom: Soil texture input datasets of sand, clay, and organic carbon
derived from the three data sources.
2.3 The Land Surface Model
2.3.1 The Community Land Model
The Community Land Model v.5 (CLM5) used in this study is the latest
version of the land component in the Community Earth System Model (CESM)
as described in detail by D M Lawrence et al. [2019]. CLM5
simulates land surface energy fluxes as well as hydrological,
biogeophysical, and biogeochemical processes that are driven by
atmospheric input variables in combination with soil and vegetation
states and characteristics [D Lawrence et al. , 2018]. These
processes are simulated on different subgrid units within a grid cell.
Subgrid units include (1) the land unit defining the land use category
(e.g., vegetated, urban, crop), (2) the column that is represented by 20
soil and 5 bedrock layers and resolves state variables and fluxes of
water and energy in the soil, and (3) the patch level capturing
biogeophysical and biogeochemical differences between plant functional
types (PFTs) (e.g., broadleaf deciduous forest, evergreen shrub, maize,
soy). The one-dimensional multilayer vertical water flow in the soil is
simulated using a modified Richards equation [Dingman , 2015].
Soil hydraulic parameters for these calculations are derived from
pedotransfer functions of sand and clay [Clapp and Hornberger ,
1978; Cosby et al. , 1984] and organic properties of the soil
[D Lawrence and Slater , 2008]. With version 5 of CLM, a plant
hydraulic stress routine was introduced that uses a simple hydraulic
framework to model water transport along a water potential gradient from
soil via plant to atmosphere [Kennedy et al. , 2019]. The new
configuration replaces soil potential with leaf potential as the basis
for plant water stress while root water potential is used to drive root
water uptake. A new biogeochemistry and crop module, BGC-Crop, enhanced
the representation of major crop functional types and land management
practices such as irrigation and fertilization. Unlike natural
vegetation that competes for water and nutrients, crops operate on
separate soil columns that may be irrigated or non-irrigated, thus
allowing for differences in land management [D M Lawrence et
al. , 2019].
The recent development of CLM5-FruitTree enables the simulation of
deciduous fruit trees and associated management practices in CLM5. The
main features of the new sub-model include (1) a perennial phenology
routine that allows the woody plant parts to remain on the orchard for
several years, (2) carbon storage dynamics that enable the regrowth of
annual plant parts, (3) an adapted carbon and nitrogen allocation, and
(4) the description of typical management practices such as
transplanting, pruning, and orchard rotation. Additionally, a new apple
plant functional type was parameterized while fertilization and
irrigation use the default CLM5 schemes. The complete model development
of CLM5-FruitTree is described in Olga Dombrowski et al.[2022].
2.2.3 Irrigation module in CLM5
Irrigation is performed individually over each irrigated soil column and
responds dynamically to SM based on a daily check at 6 am. If crop leaf
area is non-zero and if the available soil water over a specified
irrigation depth \(z_{\text{irrig}}\) (=0.6 m by default) is below a
defined threshold, irrigation is triggered. The irrigation amount is
based on the SM deficit (𝐷𝑖𝑟𝑟𝑖𝑔) that is calculated over\(z_{\text{irrig}}\):