Soil moisture is projected to decrease in many regions in the 21st century,
exacerbating local temperature extremes. Here, we use sensitivity experiments
to assess the potential of keeping soil moisture conditions at historical
levels in the 21st century by “recycling” local water sources (runoff and a
reservoir). To this end, we develop a “land water recycling” (LWR) scheme
which applies locally available water to the soil if soil moisture drops
below a predefined threshold (a historical climatology), and we assess its
influence on the hydrology and extreme temperature indices. We run ensemble
simulations with the Community Earth System Model for the 21st century and
show that our LWR scheme is able to drastically reduce the land area with
decreasing soil moisture. Precipitation responds to LWR with increases in
mid-latitudes, but decreases in monsoon regions. While effects on global
temperature are minimal, there are very substantial regional impacts on
climate. Higher evapotranspiration and cloud cover in the simulations both
contribute to a decrease in hot temperature extremes. These decreases reach up to about
Land water plays an important role in the development of temperature extremes and heatwaves.
Changes in soil moisture (SM) can alter the amount of water that is available
for evapotranspiration (ET), affecting local climate through its impact on
the energy and water cycles
Projections for the 21st century show decreasing SM in many
mid-latitude regions
More reality-based experiments on the potential effects of the land water
cycle on climate can be obtained using simulations assessing the influence of
irrigation. Irrigation is a land management practice that applies water to
the soil, elevating SM levels. Therefore, irrigation does not only help
sustain global food production, by providing agricultural crops the necessary
water to grow, but it also influences local weather and climate. There are a
number of studies investigating the impact of irrigation on climate utilizing
global climate models
However, irrigation uses large quantities of water. The estimated global water
consumption for irrigation has risen from approximately 600 km
In this study we assess if it is possible to sustain historical SM levels in the 21st century without over-extracting local water resources. For this purpose, we develop a “land water recycling” (LWR) scheme that irrigates the soil if SM levels fall below late 20th-century conditions. The LWR scheme only uses local water sources; thus, water is only applied to the soil if it is available from runoff, and, potentially, a reservoir. We investigate if global-scale LWR is able to keep SM conditions at late 20th-century levels under future climate conditions and gauge its potential to mitigate local temperature extremes.
The Community Earth System Model
CLM4.0 is a third-generation land surface model
The aim of “land water recycling” (LWR) is to keep SM conditions above a
certain threshold, but only if water from local sources is available.
Therefore, we develop a LWR scheme by extending an existing SM prescription
module
Land water recycling (LWR) scheme, list of experiments, and area
where LWR is applied in the RES50_CROP experiment (see
Sect.
We use CESM to generate four climate ensembles that have three members each. The
first ensemble is a reference simulation (REF), forced with historical
“all forcing” conditions from 1850 to 2005, and prolonged until 2099 with
the Representative Concentration Pathway 8.5 scenario
We conduct three further ensembles, RUNOFF_ONLY, RES50, and RES50_CROP,
which are also summarized using the term “experiments” (EXP). The experiments are
branched off the reference simulations in 1950. In these simulations we apply
the above-mentioned LWR scheme. The SM target (Sect.
In the first sensitivity experiment, RUNOFF_ONLY, water is only taken from
the runoff in the same grid cell, i.e. the reservoir capacity is 0 mm
(Fig.
All analyses are carried out on the pooled ensemble members, either using annual values or the mean over the respective regions' warm season, defined here as the 3 warmest consecutive months. We determine the warm season in REF for a historical period (1971–2000). The warm season used generally corresponds to summer in mid- and high-latitude regions (Fig. S1 in the Supplement). However, other time frames are found in some other regions, e.g. in the tropics.
In our analysis we focus on the end of the 21st century and
calculate 30-year climatologies for the period from 2070 to 2099. In light of the emerging
literature on the effects of limiting global warming to 1.5 or
2.0
To assess the influence of LWR on temperature extremes, we compute three
indices from the daily model output. These indices are as follows: (i) the hottest daytime
temperature of the year (TXx), measuring the intensity of heat extremes;
(ii) the percentage of days that exceed the 90th temperature
percentile (TX90p), i.e. the frequency of heatwaves; and (ii) the duration
of the longest heatwave per year when the 3-day running mean exceeds
the 90th temperature percentile (HWD), which is a measure of heatwave
length. The 90th temperature percentile is calculated using
the method from
Where appropriate, we test for significance with a Wilcoxon–Mann–Whitney
The influence of LWR on the surface temperature (TS) can be
investigated with the help of the energy balance decomposition
The LWR scheme only applies water to the soil if SM falls below the late
20th-century climatology. Therefore, it is of interest to
assess the SM development in the 21st century. For the warm
season, CESM projects a strong decrease in surface SM, especially in Europe,
North America, South Africa, and north-eastern South America, whereas most
other regions show a small to moderate increase
(Fig.
Projected change of SM in the topmost 10 cm of the soil relative to the soil moisture climatology (1971 to 2000) in REF. Only the warm season (3 hottest consecutive months) is considered (Fig. S1).
LWR is able to substantially reduce the area with a negative SM trend
(Fig.
LWR is not only expected to influence SM, but also other components of the
hydrological cycle. In our analysis we will concentrate on precipitation, ET,
runoff, and the reservoir that is introduced. Comparing precipitation at the end of
the 21st century between the experiments and REF reveals
some distinct patterns (Fig.
Difference between EXP and REF for precipitation (Precip),
evapotranspiration (ET), and runoff. The fourth row shows the mean reservoir
state. Hatching in the first three rows indicates grid cells with significant
changes (Wilcoxon–Mann–Whitney
The regions with decreasing precipitation coincide to a large degree with
monsoon regions
Mean annual differences between EXP and REF for precipitation, ET,
and runoff (in km
An increase in ET is expected when adding water to the soil, which is
confirmed in Fig.
Runoff is the only source of water for the LWR, for both direct water
application and to fill the reservoir; thus, we generally expect a decrease.
Indeed, global runoff decreases by approximately two-thirds of the annual
discharge of the Amazon River
The long-term (2070 to 2099) average of the reservoir implies that the
reservoir is either (almost) full or empty (Fig.
Next we turn our attention to the temperature effect of LWR. The global
annual land temperature is reduced by
The effect of LWR on extreme temperature indices is shown in
Fig.
For Europe and central North America, in comparison, REF indicates wet
soils in the historical period and a strong drying in summer in the next
century (Fig. S6). LWR is able to overcome this drying, especially when
allowing for storage of water in a reservoir, causing the strong cooling in
these regions. In historical irrigation simulations
Difference between REF and RUNOFF_ONLY (left column panels), RES50
(central column panels), and RES50_CROP (right column panels) for temperature
indices. The top row represents TXx (intensity), the middle row represents TX90p (frequency), and the bottom row
represents HWD (duration). Hatching indicates significant grid cells
(Wilcoxon–Mann–Whitney
For the next two indices (
To better understand the LWR-induced change in temperature, we make use of
the energy balance decomposition (Eq.
The seasonal cycle and annual mean (Ann) of surface temperature anomalies and energy balance decomposition for central North America (CNA), South Asia (SAS), and South Africa (SAF). Grey shading indicates the warm season. TS stands for surface temperature, SWnet denotes net short-wave radiation; LWin denotes incoming (downward) long-wave radiation; LH and SH denote the latent and sensible head flux, respectively; and R is the residual term, which includes the ground heat flux.
SAF shows almost no seasonal cycle in TS and the contribution of the
radiative- and land-terms of the energy balance seems to be more evenly
distributed (Fig.
We have shown that the LWR scheme developed in this study reduces extreme temperatures;
however, this raises the question as to whether it was also able to offset half a degree increase in the global mean
temperature. We answer this question in Fig.
Offset of half a degree additional global mean warming by the LWR
experiments, see Eq. (
In RUNOFF_ONLY, LWR is able to offset the additional warming in parts of Eurasia, the Americas, and Australia, but not in Africa and South Asia. Allowing for water storage (RES50) leads to a larger area where the LWR-induced cooling dominates over global mean warming. However, temperatures in Africa and the southern parts of Asia still remain warmer. Finally, in RES50_CROP the cooling effect is mostly restricted to the LWR-areas in central North America and Europe.
Thus, our LWR scheme is able to locally offset the warming from half a degree
additional warming. However, it does not change the general warming trend
due to rising greenhouse gases, which are almost the same in the LWR
experiments and REF (Fig. S8). This finding should be taken with caution, as
we prescribe SSTs which will dictate the global mean warming. Nonetheless,
they are in accordance with a similar study using an interactive ocean which
also showed that the trend in regional temperatures is similar with and
without irrigation throughout the 21st century
In this study we used idealized climate model experiments to study the effect of sustainable global-scale land water management and its impact on temperature extremes. To this end, we developed a land water recycling (LWR) scheme that applies water to the soil if (i) it is drier than in the 1971 to 2000 median soil moisture climatology and if (ii) water is available from local sources.
We compute four sets of climate model experiments with the Community Earth System Model with three ensemble members in each. The four ensembles comprise a reference simulation and three sensitivity experiments including LWR. In the first sensitivity experiment, LWR only applies water to the soil if runoff from the same time step is available. In the second sensitivity experiment water is also taken from runoff, but a reservoir with a capacity of 50 mm is additionally available such that e.g. surplus water that accumulates during the wet season can be used for LWR in summer. Finally, the third sensitivity experiment also uses runoff and a reservoir as water sources, but only applies water in areas with a crop fraction of at least 10 %.
We have shown that LWR is able to maintain soil moisture conditions at late
20th-century levels for a large part of the global land
area. However, LWR also has a marked impact on the hydrological cycle: it
leads to an increase in precipitation in the mid- and high- latitudes, which is
beneficial for areas where a precipitation decrease is projected for the next
century. However, averaged over the global land area, this local increase is
overcompensated for by a reduction in precipitation in monsoon regions. As
expected, LWR leads to a large-scale increase in ET, due to
higher soil moisture levels, and a decrease in runoff, as this is the only
source of water applied. Further, the reservoir implemented is either “full”
(
LWR cools mean land air temperatures, but overall the effect is relatively
small (
Adding a reservoir generally leads to more LWR and thus strengthens the response of the climate. This is clearly visible in central and southern Europe, and central North America. Precipitation projections for the 21st century indicate a strong decrease in these regions during the warm season, but not for the whole year, which renders the reservoir especially effective. Restricting LWR to regions with at least 10 % crops, in comparison, mostly restricts the influence on these regions.
While applying a water management scheme that affects the whole land area is
certainly unrealistic, these sensitivity experiments can place an upper limit
on the potential of LWR to mitigate climate change. Certain irrigation
modules impose no limit on the water available for irrigation
The LWR approach is sustainable in the sense that it does not use more water
than locally available from runoff, and it does not lead to the depletion
of groundwater reservoirs. However, our scheme imposes a large stress on
runoff, leaving no residual flow in some regions. In practice this would have
devastating ecological implications and would dramatically reduce river sediment
transport
Overall, we were able to show that sustainable land water management is theoretically able to keep SM conditions at late 20th-century levels. Our study provides a new perspective on how land water can influence local and regional climate. While LWR only has a small influence on mean temperatures, it leads to a substantial decrease in extreme temperatures; thus, it can be seen as potential tool for local mitigation of climate change.
The code used is available at
The supplement related to this article is available online at:
MH performed the majority of the analyses and wrote the paper with input from the co-authors. All authors participated in the design of the experiments and the discussion of the results.
The authors declare that they have no conflict of interest.
We thank Urs Beyerle for support with CESM. We acknowledge partial support from the European Research Council (ERC) DROUGHT-HEAT project (contract 617518). Edited by: Axel Kleidon Reviewed by: two anonymous referees