Figure 7: Seasonal mean soil
moisture, and evapotranspiration sums in the PHO catchment, averaged
over the period 2016-2022.
3.2.2 Simulated spatial patterns
Figure 8 shows average and standard deviation of the 7-year simulation
period for irrigation, SM, yield, and CWUE for all apple orchards in the
PHO, between 2016 and 2022. Modeling results show a clear spatial
pattern that is driven by climatic conditions following the topographic
gradient (Figure 1) on the one hand and soil characteristics on the
other hand (Figure 2). Average yearly irrigation requirements range
between 400 and 450 mm in the plain. The highest values are found in the
southeast while considerably lower values occur at higher altitudes in
the northern part of the catchment (<200 mm). Harvest values
show a similar pattern because cooler temperatures and lower incoming
radiation in the northern part of the catchment result in lower crop
productivity and thus smaller yields (16-38 t ha-1)
compared to the plain where yields are around 50 t
ha-1 without much spatial variability. In addition to
lower crop productivity and thus lower crop water demands, spatial
variability in irrigation requirements results from the higher
precipitation in the upper parts of the catchment that further reduces
the need for irrigation as well as soil textural differences. The latter
is most evident in the southern part of the catchment where the higher
clay content and the consequently higher water holding capacity of the
soil result in increased evaporation (not shown). This in turn generates
a greater irrigation demand resulting in slightly lower CWUE of orchards
planted on these soils. Soil textural differences are also reflected in
the SM plot where areas with a higher percentage of clay or organic
matter show higher SM values than areas with sandier soils or soils that
are lower in organic matter. CWUE ranges from 57-65 kg
ha-1 mm-1 in the plain to 35-45 kg
ha-1 mm-1 in the northern part of
the catchment and largely reflects the spatial patterns of irrigation
and harvest whereby high irrigation requirements and low harvest lead to
low CWUE. Inter-annual variability (standard deviation plots) within the
catchment shows similar patterns for irrigation, harvest, and CWUE and
is higher in the northwestern part of the catchment. The higher
variability was driven by local temperature differences in some years
that delayed the onset of the growing season up to 14 days compared to
the remaining orchards. Inter-annual variability of SM is generally low
without a distinct spatial pattern.