The humid tropics are exposed to an unprecedented modernisation of agriculture involving rapid and mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability which controls habitats, water resources, and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydrometeorological variables has been operating in several headwater catchments in tropical southeast Asia since 2000. The GR2M water balance model, repeatedly calibrated over successive 1-year periods and used in simulation mode with the same year of rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses, and trend detection tests allowed causality between land-use changes and changes in seasonal streamflow to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow led to intricate streamflow patterns: pluri-annual streamflow cycles induced by the shifting system, on top of a gradual streamflow increase over years caused by the spread of the plantations. In Vietnam, the abandonment of continuously cropped areas combined with patches of mix-trees plantations led to the natural re-growth of forest communities followed by a gradual drop in streamflow. Soil infiltrability controlled by surface crusting is the predominant process explaining why two modes of afforestation (natural regeneration vs. planting) led to opposite changes in streamflow regime. Given that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.
Although the humid tropics exhibit the highest rate of deforestation and biodiversity losses globally (Keenan et al., 2015; Hansen et al., 2013; Bradshaw et al., 2009), new forests are regenerating on former agricultural and degraded lands, and tree plantations are being established for commercial and restoration purposes (Miura et al., 2015). Forest regrowth is either cyclic like in shifting cultivation systems (Ziegler et al., 2011; Hurni et al., 2013) or more permanent. The latter, afforestation, is the production of forest over an area of open land either by planting or by allowing natural regeneration. If appropriately managed, forest restoration, or afforestation, can lead to biodiversity enhancement (Chazdon, 2008), not only in the forested area but also farther downstream, in response to modified hydrological processes at the hillslope and catchment levels (Konar et al., 2013). Although important for a sustainable management of headwater catchments, the current understanding of hydrological processes altered by land-use changes remains limited in the tropics (Sidle et al., 2006). Reasons include the scarcity of long-term field monitoring (Douglas, 1999; Wohl et al., 2012) and several factors confounding causalities between land use and hydrological changes: mixed land-use patterns, climate variability, and catchment size (Beck et al., 2013; van Dijk et al., 2012). While it is widely and independently recognised that evapotranspiration is a central driver of basin annual water yield (Brown et al., 2005), changes in soil infiltrability also control groundwater recharge and water uptake by roots (Beck et al., 2013; Bruijnzeel, 2004). While in most cases, afforestation will reduce streamflow (Brown et al., 2005; Calder, 2007), the opposite or the absence of significant hydrologic changes are observed in some instances (Wilcox and Huang, 2010; Hawtree et al., 2015). The lack of an unequivocal hydrological response to afforestation feeds controversies around the role of forests in controlling river flows (Andréassian, 2004) and highlights the need for further research (Calder, 2007). A few studies have attempted to predict the catchment-scale hydrological effects of land-cover changes on streamflow in the humid tropics, mainly from model-based simulations of land-use change scenarios (Thanapakpawin et al., 2006; Guardiola-Claramonte et al., 2010; Homdee et al., 2011). Hydrological assessments based on actual data are rare in the humid tropics (Wohl et al., 2012) and often confined to the plot level (Ziegler et al., 2004; Podwojewski et al., 2008; Valentin et al., 2008a; Patin et al., 2012).
Two main approaches are usually deployed to assess how land-use changes alter hydrology. Paired catchment studies establish statistical relationships for outflow variables, during a calibration period, between two neighbouring catchments ideally similar in geomorphology, area, land use, and climate. Following this calibration, land-use treatments are applied to one catchment and changes in the statistical relationships are indicative of the land treatment effect on hydrology. Important limitations of this approach are the relatively few samples used for model development, and the spatial variability of rainfall events between the two catchments (Zégre et al., 2010). A second approach involves the calibration of a rainfall–runoff model in one single catchment. The model is first calibrated before a land-cover treatment occurred. The model is then used as a virtual control catchment along with rainfall observed after the land-cover treatment, in order to reconstitute runoff as if no change in the catchment had occurred. An underlying assumption for this approach is that the catchment behaviour is stationary in both the pre-treatment and post-treatment periods. This assumption is seldom tested. In addition, very few studies have tested the statistical significance of changes in the relationship between rainfall and runoff (Zégre et al., 2010).
The objectives of our research were to
Monitor inter-annual and long-term changes in land use and hydrology in
two headwater catchments in tropical southeast Asia, one exposed to a
gradual conversion of rainfed rice-based shifting cultivation to teak
plantations in Laos, and one subject to natural forest regrowth following
the abandonment of intensively cultivated hillslopes with cash crops and
patches of mixed-trees plantations in Vietnam. Use a conceptual monthly lumped water balance model repeatedly
calibrated over successive 1-year periods and used in simulation mode with
specific rainfall input to generate cross simulation matrices
(Andréassian et al., 2003). These matrices are used to isolate the
hydrological effect of rainfall variability from that of other environmental
changes (e.g. land-use change, in this article) in each study catchment. Apply correlation analyses and a non-parametric trend detection test to
streamflow reported in the cross simulation matrices, to investigate and
quantify causal relationships between land-use changes and changes in the
hydrological behaviour of the study catchments, and assess whether the
hydrological changes are statistically significant over the whole study
period. Compare the effects of forest plantations and natural forest regrowth on
streamflow in the two study catchments.
The two study catchments of the MSEC network and their land use in 2013.
Monthly rainfall, runoff, and
The two study catchments (Fig. 1) are part of a regional monitoring network
named “Multi-Scale Environmental Change” (MSEC,
Catchments characteristics.
The Houay Pano catchment in Laos is located about 10 km south of Luang
Prabang city. It is representative of a landscape dominated by shifting
cultivation, the principal activity in the uplands of northern Laos. The
catchment was first cleared of semi-deciduous forest in the late 1960s (Huon
et al., 2013) and used for shifting cultivation (crop–fallow rotation). In
this system, one annual crop comprising mainly rainfed rice (
The Dong Cao catchment is located in northern Vietnam, about 50 km southwest
of Hanoi, along the eastern side of the Annamite Mountain range. The
catchment was covered by lowland primary forest prior to 1970. Paddy rice and
arrowroot (
Data were collected by IRD (Institut de Recherche pour le Développement)
and the national agricultural research institutions from April 2001 to March
2014 in Laos, and from April 2000 to March 2014 in Vietnam. They include
records of daily rainfall, reference evapotranspiration (
Land use was mapped annually for 13 years (April 2001–March 2014) from
detailed field surveys undertaken each year in October–November, after the
harvests of annual crops, when fields are clearly marked and easily
accessible without damaging crops. A combination of GPS and theodolite survey
points were used in the field to map boundaries between land-use units.
ArcMap 10.0 was used to estimate the proportion of each land-use unit in each
catchment. The mapping accuracy of land-use boundaries is estimated to be
within
In Laos, the unit “Annual crops” includes rainfed upland rice,
Job's tears and maize; “Forest” includes patches of remaining
forest, either mixed deciduous or dry Dipterocarp; “1-year fallow”
and “2- to 12-year fallow” form two distinct land-use units due to
differences in soil surface crusting rates and associated hydrodynamic
conductivity (Ziegler et al., 2004); Teak plantations are often associated
with annual crops during the first 2 years after planting
(“Teak
In Vietnam, the unit “Forest communities” combines abandoned
farmland that has developed into an open forest, usually after 5 years of
undisturbed growth, and patches of more developed secondary forest;
“Mixed-trees plantations” includes acacia, eucalyptus, cinnamon,
and fruit trees, both young and mature. These plantations have developed an
understorey of natural vegetation; “Forbs” are abandoned farm
lands covered by a dense herbaceous cover of perennial dicots and grasses,
usually developed within 5 years since the last cropping; “Annual
crops” include cassava and maize; “Fodder” corresponds to the
planted exotic grass
The two-parameter monthly lumped water balance model GR2M was used to
investigate changes in the hydrological behaviour of the two study
catchments. This model was empirically developed by Mouelhi et al. (2006)
using a sample of 410 basins under a wide range of climate conditions. GR2M
includes a production store and a routing store. The model estimates monthly
streamflow from monthly areal rainfall and monthly
In a given matrix, each column
Cross-simulation matrix. Here,
Following the approach proposed by Andréassian et al. (2003), the statistical
significance of gradual changes in catchment behaviour was calculated using
cross-simulation matrices similar to the one illustrated in Fig. 3.
Each of the two original matrices was resampled 10 000 times by permuting
columns. For each original and permuted matrix, the statistic
Annual rainfall, runoff, and runoff coefficient measured in Houay
Pano
Annual rainfall and runoff variations are consistently correlated in Laos
(
Houay Pano catchment, Laos. Wet season
Annual values of
Dong Cao catchment, Vietnam. Wet season
Correlations between simulated streamflow and land-use types.
Panels
Over the first sub-period (2002–2006), on average, an increase (decrease) of
Annual values of
Figures 5 and 7a, b indicate that catchment streamflow is predominantly produced by the following land-use units: annual crops, 1-year fallow, and teak plantations while 2- to 12-year fallow, forest, and banana plantations make a comparatively lower contribution to annual streamflow production. In agreement with these observations, Ribolzi et al. (2008) determined a negative correlation between the percentage area of total fallow and annual runoff coefficients in the same catchment over the period 2002–2006. However, the authors could not ascertain the causality between these two variables because the possible effect of rainfall variability (gradual decline of annual rainfall from 2002 to 2006, cf. Fig. 4a) on streamflow was not isolated from that of land-use change (gradual increase of total fallow areas from 2002 to 2006, cf. Fig. 5c).
The contrasting hydrological behaviour of areas under annual crops and 1-year
fallow, on the one hand, and areas under 2- to 12-year fallow, on the other
hand, observed at the catchment level, are consistent with local
observations. Using several 1 m
The hydrological processes involved in the conversion of the rice-based
shifting cultivation system to teak plantations are less intuitive. Teak
trees can develop relatively high leaf area index (Vyas et al., 2010), deep
and dense root systems (Calder et al., 1997; Maeght, 2014), i.e. traits
consistent with a high water uptake by evapotranspiration. To that extent,
their hydrological impact should be similar to that of 2- to 12-year fallow during the wet
season. However, (1) under young teak trees, the inter-row area
is cultivated with annual crops with high rate of soil surface crusting; (2) the
large leaves of mature teak trees concentrate rainfall into big drops
that hit the soil with increased kinetic energy hence forming surface crusts;
and (3) most farmers intentionally keep the soil bare under mature teak trees
by recurrent burning of the understorey. These three facts create the conditions for intense
erosion that induces features such as gullies, raised pedestals, and root
exposure. Suppression of the understorey led to the formation of impervious
crusts that limited infiltration and in turn increased Hortonian overland
flow and erosion, as typically observed in teak plantations where fires are a
common phenomenon (Fernández-Moya et al., 2014). These processes were
quantified at the 1 m
No local measurement of infiltrability and soil surface crust was performed under the natural forest in the Houay Pano catchment. Therefore, it is not possible to conclusively prove their contribution to the catchment outflows. However, correlation analyses showed that this land-use unit behaves hydrologically like 2- to 12-year fallow (cf. the position of this land-use unit above the black solid-bold curve in Fig. 5c). This is in accordance with Brown et al. (2005) and with our findings in Vietnam (cf. Sect. 4.2, Figs. 6 and 7c, d), showing that sparser (denser) natural vegetation cover increases (reduces) streamflow. Finally, it should be noted that the area covered with banana trees remained stable over the study period and had no discernable effect on streamflow variations.
Figures 6 and 7c, d indicate that catchment streamflow is predominantly
produced over herbaceous land-use units (annual crops, forbs, and fodder),
while tree-based land-use units (mixed-trees plantations and forest
communities) make a comparatively lower contribution to streamflow (cf. the
location of these groups of land-use units below and above the black solid-bold
curve in Fig. 6c, respectively). These differences are consistent with local observations. Deploying
several 1 m
Two types of land-use successions occurred in the Dong Cao catchment: (i) from annual crops and fodder to forbs and finally to forest communities and (ii) from mixed-trees plantations to forest communities (Fig. 6c). These land-use changes are the result of afforestation by natural regeneration in both abandoned fields and neglected tree plantations, respectively. As indicated in Podwojewski et al. (2008), these natural successions are converging on lower surface runoff coefficients caused by increased infiltrability, allowing the evapotranspiration of larger volumes of sub-surface and ground water through denser and deeper root systems and denser tree canopy (Dunin et al., 2007; Ribolzi et al., 2008). This explains the decrease in simulated wet and dry season streamflow at the catchment level (Fig. 6a, b) from 2002 to 2013. The visual comparison of the simulated streamflow time series (Fig. 6a, b) with the time series of the cumulative percentage area of the herbaceous land-use units (e.g. the black solid-bold curve in Fig. 6c) indicates a 1-year delay in the response of seasonal streamflow to land-use changes, which is confirmed by correlation analyses (Fig. 7c, d). This delay is already known from a number of catchment experiments globally. Brown et al. (2005) showed that annual water yield altered by forest regrowth experiments takes more time to reach a new equilibrium, compared to deforestation experiments that usually induce quicker hydrological responses. In Laos, no time lag was observed between land-use changes and changes in simulated streamflow (Fig. 5) because this temporality was already accounted for in the difference made between 1-year fallow and 2- to 12-year fallow exhibiting contrasting soil surface crusting rates and infiltrability.
The reduction of the Dong Cao catchment water yield over the full study
period is equivalent to a reduction of about 165 000 m
The dynamics of land-use changes in the Houay Pano catchment, Laos, involved
cyclic patterns (landscape dominated by shifting cultivation and teak
plantation expansion) whose hydrological effects would remain undetected if
we had restricted our analysis to the statistical detection of gradual and
unidirectional change in the rainfall–runoff relationship
(
Two main types of land-use change at the scale of the Houay Pano catchment had different
hydrological impacts: (i) the transition from (2- to 12-year fallow and
forest) to (annual crops and 1-year fallow); (ii) the transition from (2- to
12-year fallow and forest) to (annual crops, 1-year fallow, and teak
plantations). The first (observed over 2001–2006) induced increases in
simulated seasonal streamflow lower than those induced by the second
(observed over 2006–2013), as illustrated by the different slopes of the
regression lines in Fig. 7a, b. Thus, teak plantations, recently introduced
to replace traditional rice-based shifting cultivation systems, are
generating more runoff than was generated by annual crops and 1-year fallow.
This difference did not appear in the average values of infiltrability
obtained by Patin et al. (2012) at the microplot level: 18 and 19 mm h
The hydrological effect of this modern land conversion in Laos is of the same
magnitude (but in the opposite direction) as that caused by the conversion of
herbaceous cover (annual crops, forbs, and fodder) to naturally
regenerating tree-based covers in Vietnam (mixed-trees plantations and forest
communities). In the two countries, the switch from herbaceous cover
(including teak tree plantations in Laos) to old fallow and/or forest over
1 % of the catchment area translates into reductions of wet and dry
seasons' streamflow of about 10–12 mm and 1.5–3.5 mm, respectively (cf. the
coefficients of the linear regressions in Fig. 7a, c and b, d, respectively).
Assuming the linearity of these relationships, the average difference between actual annual
evapotranspiration of the herbaceous cover (including teak trees in Laos) and natural tree-based cover ranges
between 100
A two-parameter monthly lumped water balance model was used to investigate
the relationship between land use and catchment hydrology. This approach
presents some limitations. For instance, land-use changes occurring within or
outside of the riparian area and their hydrological effects were not
differentiated. The spatial patterns of the land-use mosaics (e.g. area,
layout, and connectivity of the patches) were not accounted. This
simplification limits our understanding of the processes underlying the
rainfall–runoff transformation. However, the model efficiently captured the
gradual changes in the catchments' behaviour (mean values of
It could be argued that 1-year calibrations are too short for the model to accurately capture the hydrological behaviour of the catchment. This statement would be valid in the context of a more classical split-sample test including a calibration and a validation period where the model is used as a predictor. This procedure assumes that the catchment is hydrologically stable over these two sub-periods. In our approach, the water balance model was used to capture gradual changes in hydrological behaviour in order to verify if these changes are caused by actual changes in land-use conditions. With this aim, minimising the duration of the calibration periods to 1 year allowed maximising the dependency between the model parameters and the corresponding land-use patterns mapped annually. This approach proved to be appropriate given the high inter-annual variability of land use (Figs. 5c and 6c), and the significance of the correlations between land use and streamflow simulated with the different calibrated models (Figs. 5, 6, and 7). However, a 1-year calibration may result in a model that performs well under the specific climate conditions of the calibration year only. Simulation biases usually increase when the model is run under climate conditions different from calibration conditions (Coron et al. 2012), thus possibly hampering the detection of the hydrological changes illustrated in Figs. 5 and 6. To quantify this bias, GR2M was calibrated over the 2-year period (2012–2013) in the Dong Cao catchment where land use remained relatively stable between 2011 and 2013 (Fig. 6c). The rainfall years 2012 and 2013 correspond to the median (1421 mm) and the wettest (1938 mm) years, respectively, of the study period (2002–2013) (Fig. 4). Therefore, this two-year period exhibiting stable land use but contrasting rainfall conditions is well suited to investigate the effect of rainfall variability and calibration duration on model efficiency. The mean relative difference between streamflow simulated by this model and by the models calibrated over the 1-year periods 2012 and 2013 (the three models use the same 2012 year as rainfall input) approximates this simulation bias which was found to be higher for the wet season (20 %) than for the dry season (2 %). Overall, these biases are negligible compared to the major hydrological changes observed in the two study catchments: 67 % wet season streamflow reduction and 84 % dry season streamflow reduction over the study period in the Dong Cao catchment; 100 % wet season streamflow increase and 650 % dry season streamflow increase in the Houay Pano catchment between 2007 and 2011. In contrast, wet season streamflow over the period 2002–2006 in the Houay Pano catchment (Fig. 5a) exhibits the lowest inter-annual variations for a 5-year period in the study catchments, with a coefficient of variation (11 %) lower than the 20 % bias estimated for the wet season simulations, indicating a possibly significant modelling artefact. However, these streamflow variations are significantly and consistently correlated to land-use change over this short period (Fig. 7a), suggesting negligible biases even for these slightest streamflow variations. The main discrepancy between simulated streamflow and land use was observed during the 2009 wet season in the Dong Cao catchment (Fig. 6). In 2009, simulated streamflow is equivalent to about one-third of that in 2008 and 2010, while no major change in land use apparently explains this drop. This discrepancy could originate from a simulation bias because 2009 was the driest year of the study period (Fig. 4).
Our results show that the land-use effects on soil surface properties and
infiltrability, previously quantified in 1 m
According to the most recent Global Forest Resources Assessment (FAO, 2015), Laos and Vietnam are listed among the 13 countries globally which were likely to have passed through a national forest transition between 1990 and 2015, with a switch from net forest loss to net forest expansion (Keenan et al., 2015). Our analysis exemplifies the diverse impacts this forest expansion can have on streamflow, and how it can lead to extreme, yet opposite, hydrological changes, depending on how the newly established tree-based cover is managed. The conversion of rice-based shifting cultivation to teak plantations in Laos led to increased seasonal streamflow. The conversion of annual crops and mixed-trees plantations to naturally re-growing forest in Vietnam led to decreased seasonal streamflow. Considering that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.
The data set used in this analysis,
including hydro-meteorological records and land-use maps, is available at
This work was funded by the French watershed network SOERE-RBV (réseau des bassins versants), the French Observatory for Sciences of Universe (Observatoire des Sciences de l'Univers), the CGIAR research program on Integrated Systems for the Humid Tropics, and the French ANR TECITEASY (ANR-13-AGRO-0007). The authors gratefully acknowledge the Institute of Research for Development, the International Water Management Institute, the Soils and Fertilizers Research Institute (Vietnam), and the Agriculture Land-Use Planning Center (Laos).Edited by: G. Jewitt